Ergonomically, it is a well-designed (although excessively lightweight) mouse. However, my Sensei had double the weight, but it didn't seem difficult to use at all.

The two main buttons have a strong clicking sound and are somewhat strange compared to my old mice from Roccat, Steelseries. Because the top cover is not seamless with the rest of the chassis, the left-right buttons are a bit loose and wobbly.

I'm used to mouse wheels making noise with each step, but this one feels like it doesn't even make contact. To be honest, I don't really like that.

The position of the side buttons is quite convenient for the thumb on one side, but I will need some time to get used to the right side, as it is slightly further back compared to the Steelseries I had.

Synapse is very minimalistic, I would say, and even for the simplest macro, you need to install the "MACRO" module. It consumes a lot of memory compared to the Steelseries Engine. Razer Central, Razer Synapse, GameManagerService, and yet it doesn't do anything special unless you also install some bloatware modules, e.g. for RGB control, which, jokingly, doesn't really show the differences in color tones... although those are irrelevant to me.

Since I got it for €49 on Black Friday, it's still good. But for the €80 they sell it for, not even as a joke...

P.S. I suppose I will get used to it soon, but I regret not getting the Sensei Ten that I already "knew."

This particular mouse arrived in my hands yesterday, and coming from a DeathAdder Left Handed Edition, I can say that I am very satisfied. Anyone who is left-handed knows how difficult it is to find a mouse that suits them. Unfortunately, the options for dedicated left-handed mice are now nonexistent, so you are forced to go for an ambidextrous one. Some initial observations:

* Ergonomics: My biggest fear before purchasing it. Going from a left-handed to an ambidextrous mouse, I was worried that it wouldn't suit me, but the Viper pleasantly surprised me. The curves in the middle of the mouse below the side buttons are very ergonomic. It fit very well in my hand, which has a grip that is something between fingertip and claw. For palm grip, I consider it not ideal, especially if someone has large hands/fingers.

* Main Buttons (Left-Right): They are a bit "harder" to click compared to my previous mouse, and I noticed that I miss some clicks in games like LoL/Dota that require continuous and fast bursts. I will wait to see how my fingers adapt and will update the review in a month.

* Side Buttons: I had read various things online, but personally, I didn't find any particular difficulty in pressing them. On the positive side, the opposite side buttons are practically impossible to press by mistake, which, combined with the ergonomic curves below them, makes you believe that you are holding a left-handed (or right-handed, respectively) mouse and not an ambidextrous one.

EDIT (16-06-2020): My fingers have adapted to the slightly harder click of the Viper.

* Weight (69g): Compared to my previous mouse (148g), I find it excellent. Yes, it is light, but not to the point where you don't feel it in your hand.

* Extras: Very nice cable. It is braided with a fabric-like material, and when you move it, the mouse remains still.

First, I want to say for those who haven't understood that this mouse is lightweight, meaning it is light. If you are used to Razer mice that are relatively heavy like the Deathadder, this will surprise you. On the other hand, those who love lightweight mice will really like it. The mouse is indeed light (69 grams without the cable) and at first, when you lift it, you think it’s fake. Also, what I liked is that Razer did not copy other companies to make a lightweight mouse, meaning it wasn’t made with holes like the Glorious Model O etc., but it is completely closed and light (a bit heavier than those with holes). The good thing about not having holes is that it doesn’t collect dust.
In terms of design, I must say it is very nice and it is ambidextrous, meaning it works for left-handed users as well and it is truly ambidextrous because it has side buttons (2) on the right side too. It fits very well in the hand and personally, I like its sides where it is not plastic but rubberized (whatever they call it) for better grip. Also, the matte black color is very nice and combines perfectly with the glossy black designs. So, no complaints about the design!
The sensor is a high-end 5g optical sensor (pmw 3390) capable of up to 16,000 dpi (dots per inch), 450 ips (inches per second), and 99.4% tracking accuracy. It also supports up to 50G. All of this is overkill, like the dpi for example, which most people use 800-3,000. That’s why I believe the Razer Focus + sensor (20,000 dpi, 650 ips, 99.6% accuracy) found in the Viper Ultimate, Deathadder V2, Basilisk V2 etc. is unnecessary since it already has a perfect one; just if you absolutely want the best, go for the others. However, I believe it’s not worth it if you only do it for the sensor. Personally, I had no issues with the Viper's sensor and it is simply perfect (one of the best) as I have seen in reviews.
In terms of buttons, it has a total of 8 programmable buttons (left-right click, scroll wheel click, 2 left and two right side buttons, and 1 underneath that changes the dpi). The most important thing here is that the left and right clicks are not just mechanical, but optical from the new switches for Razer mice, which are activated by a beam of light passing through rather than contact, sending the signal. The benefits? It eliminates debounce delay (look it up online if you don’t know), reacts faster (0.2ms), and increases the switch lifespan from 50 million clicks to 70 million. Also, they are a bit quieter since there is no contact (they don’t click much but make more of a clack sound). The scroll wheel is very high quality and nice, it is tactile and has enough resistance, and its button is also very good. The side buttons feel nice when you press them, but they are just a bit harder to press; you need to move your thumb a bit higher than how you hold the mouse. Now, I want to say something here: I thought I would use 4 side buttons, but in the end, the 2 right ones are the same as the left ones (Mouse buttons 4,5), which means whichever you press (right or left respectively) is the same. So, the right ones are intended for left-handed users. However, you can change them via Razer Synapse 3 and set them to do various things like changing the dpi up and down. I thought of something else, is it possible to use them as 2 additional mouse buttons (as long as you find it comfortable to press them because it's a bit difficult especially the right back one). If you set the right buttons in Synapse as keyboard functions and any button you want, you can assign this button in the game and they will work. Finally, the DPI button is located underneath as esports gamers requested so it won't be accidentally pressed and cause confusion while playing. It's nice and has a little LED on top that shows which DPI profile you have (supports up to 5). Personally, I changed the profiles in Synapse from 5 to 2 (800 DPI for one specific airplane game and 1,600 for all the others, Razer's default is 1,800 but I find it a bit fast).
In terms of quality, although it's a lightweight type of mouse, it is very high quality. It doesn't have rattle and is made of sturdy plastic. Of course, compared to a regular one, if you drop it, the regular one will logically withstand more due to the less material that the Viper has, but in its use, it has absolutely no problem. Don't think it's flimsy just because it's lightweight; it's made of plastic (good plastic) like all of them, but it's very lightweight and high quality.
The feet (mouse feet) are the only thing I believe Razer should have changed, and they actually did in the Viper Ultimate and Viper Mini (as well as in the later Basilisk V2 and Deathadder V2). They are made of Teflon (PTFE) but are not 100%, which is why they are black. The 100% PTFE ones are white. Anyway, they are very good, the mouse rolls very quickly and has no issues. Just if at some point you want to change them, get 100% PTFE (white) mouse feet like Hyperglides (if they have been released for the Viper) or corepads.
The cable is exactly what it promises to be. Speedflex! It is coated like most Razer cables (if not all), but the difference is that it is very lightweight, flexible, and fast. It really does its job perfectly, it doesn't resist the mouse's movement and feels like using wireless (as long as it doesn't snag anywhere). I really liked its cable a lot.
In terms of lighting (RGB), it has a Razer logo on the back. Not over the top, just a simple RGB logo. What I liked is that when it turns off, the logo is hardly visible at all (with a flashlight it looks a bit gray) because it has two layers of material. When the LED is on, if you get close, you can see a bit that the light is coming from underneath as if it's under ice. I liked it a lot. Only that it doesn't have RGB elsewhere, but as we said, it's a lightweight mouse, at least it has a little RGB.
Finally, the packaging. Don't get confused with the box you see in the randomfrankp video, as that box was before the mouse was officially released to the market, and it was sent only to some to try it out. I'm talking about the box that opens sideways, which says "a winning choice." The regular one is what you will see in other videos and has a protective plastic cover, etc.
In conclusion, I highly recommend it for competitive gaming and especially for esports, and I am very satisfied. I think Razer's choice is between the Viper and Basilisk V2. The Basilisk V2 has a slightly better sensor (which you won't notice a difference), 109% PTFE feet (you can change them on the Viper for 6-8€), and a few more buttons that are more complex. On the contrary, the Viper has the advantage of being much lighter (69g vs 92g) and that means it moves more easily and quickly, which gives you an advantage in FPS games. Really, with the mouse pad I have (Razer Firefly V2) I just push it a little (kick it) with my finger and it moves. It moves so nicely and quickly, especially if you also change the feet to 100% PTFE.
I hope I covered everything for you. I believe it's the best gaming mouse in the 70-85€ range. I recommend it!!!

I have had the mouse for 4 months and it is perfect!!!

First of all, its weight is 69 grams, which makes it even better. The mouse buttons have a solid click. They are not mechanical like the Razer Deathadder, which makes them a bit noisy. However, I don't care about the noise because when I wear headphones, I don't hear anything.

The two clicks are slightly different from each other. I don't know if it's like that for all of them or if mine has a defect. I actually like the fact that they are slightly different because I can tell which click I'm pressing each time.

Compared to the Model-O, it is much better. The Viper may be a bit more expensive, but it doesn't have any issues like cable disconnections or breaking due to the holes on the top.

Edit after about a year:

The mouse hasn't had a single problem despite daily use in games like League of Legends and moderate abuse on the keys. Nothing has changed. The mouse works just like it did in the beginning, and I highly recommend it without hesitation

I won't say much about the positives because you can see them in other reviews. The shape and weight are AMAZING and I noticed a significant improvement in my aiming in FPS games (it's not a placebo, the difference really impressed me). I had a Deathadder before the Viper, but now I realize that its shape and weight didn't suit me. The Viper is an incredible upgrade, but unfortunately, it has some flaws.

Negatives:
1. I thought that the 4 side buttons would be recognized by games as Mouse 4, Mouse 5, Mouse 6, and Mouse 7. It turned out that both sides have M4 and M5 (it's visible in the picture). So in a game's controls menu, you can only bind 2 out of the 4 side buttons. The other 2 side buttons can only be used for macros or any other function allowed by Razer Synapse 3.0. If you don't want to use Synapse, then the mouse essentially only has 2 side buttons. Maybe this is considered a detail, but I didn't see it mentioned anywhere (not even on Razer's website) and it bothered me when I discovered it.

2. I feel that the buttons are somewhat fragile, and this worries me a bit. I've had it for 5 months now and there haven't been any issues, but I don't think it will withstand, for example, the 6 years that my Deathadder lasted.

Overall, the buttons are softer and not as clicky compared to the Deathadder 2013, but this can also be seen as an advantage because they require less force to press.

It bothers me that I'm giving it 4 stars, but there's a reason.
The first one I got was fine for the first few days, but on the 4th day, the left click started to squeak. Zegetron told me to take it to the store for a replacement. The second one I got makes a lot of noise with the clicks, the right one makes a loud "thud" every time I release it after pressing it, while the left one doesn't. Zegetron didn't even respond to the email I sent about this issue, I guess they consider it normal(?).
I describe the problems in the latest comments on this thread, with videos.
https://insider.razer.com/index.php?threads/razer-viper-makes-weird-noises/

Update 9/12/2019: Zegetron responded to the email, apologized for the delay, and told me to take it back to the store for a replacement. Hopefully the third one will be okay.

Other than that, the mouse is a great tool. It's lightweight with a flexible cable, it feels wireless. The side buttons are not easy to accidentally click, and the shape is well-designed with the right dimensions for my hand. I would have preferred mechanical switches, but oh well... I wish the click was quieter, but when you have headphones on, you don't even hear it.

I am the owner of three well-known "light-weight" mice. Logitech G Pro Wireless, Glorious Model O, and Razer Viper.

It weighs just 69 grams and has a top-notch sensor. (Of course, it should be noted that no matter how good the sensor of a mouse is, we have currently reached a performance ceiling. Whether it's Razer, Logitech, or any other brand, you won't notice a difference in reality with their mid-high end models.)

It has a flexible cable, not a Paracord, but something in between traditional flexible and Paracord. I prefer it compared to the Paracord of the Model O, which, although incredibly good, tends to fall off the desk during games like Quake and CoD where I constantly make large left and right movements, making it feel like the mouse is being pulled. It has an ambidextrous design, suitable for both left-handed and right-handed users, with buttons on both sides. Lastly, it comes with fairly thick skate pads for better gliding on the Mousepad and increased durability.

It comes with optical switches for the clicks. You can notice a difference, there is indeed a difference compared to regular switches, but in the real world, it doesn't create any advantage. However, the feeling of faster response definitely boosts the user's confidence.

Personally, my hand glides on the G Pro Wireless (although I personally modded it and now it's almost perfect, but I lost my warranty), there are reports that the Glorious Model O has issues with the clicks, but for me, the holes are definitely a concern. I prefer the enclosed shell and grips of the Viper.

P.S.: There is a well-known company behind it, so we're talking about quality support, and specifically, Razer has a Greek representation with official service.

Firstly, the finalmouse mouse does not sell for under 100-120€. Secondly, the model o is not suitable for left-handed people and the shipment takes quite a while. Thirdly, the new Razer switches reduce input lag (there is no so-called debouncing that exists in mechanical switches) and have a longer lifespan.

Anyway, the mouse is well-made and despite not having the mainstream honeycomb design, it remains lightweight. The cutout for the RGB logo is not visible when the lighting is off (which personally, I like because it can be more professional for office use). I didn't choose any other company for a lightweight mouse because I am left-handed and I was excited to see that the viper is ambidextrous. One negative... I would have liked the mouse to have stock feet similar to the Glorious hyperglides for more glide, but this is a personal opinion.

In the end, the mouse is excellent and appeals to a fairly wide audience. However, if the price is excessive for some, then a model o is just right

This mouse is one of the lightest mice in the world, weighing only 69 grams. The switches of the mouse are optical, just like in keyboards (razer huntsman and razer huntsman elite), which makes the switches among the fastest in the world. Also, because the switches are optical, the mouse is very durable, meaning it can withstand 80 million clicks. The cable is designed not to get stuck, for example, when you have a mousepad with a LED strip around it (razer goliathus chroma), where there is a small brick (on the top side of the mousepad) for the settings to pass through to the mousepad. Many times, with old mice from other companies, the cable would get stuck when the mouse was very close to the brick, but now with this new technology, it has not gotten stuck even once on the brick of the mousepad. Whoever wants and needs a gaming mouse, this is the appropriate solution. Also, personally, it seems cheap to me for the technologies it provides. (For those who don't know, optical switches mean that the switch inside operates with a laser. As soon as the switch is pressed, the laser passes to the other side, and as soon as the laser passes, you see on your screen that the command you gave is completed.)