Owner of the chair for about 3 days. Former owner of a chair brand "Whatever". 5 stars and if I could give more, I would. More details below:
1. Assembly: The chair comes in 8-10 pieces and is very easy to assemble with 10 simple steps, which are explained by the very detailed instructions easily, simply, without any confusion or searching. My wife did the assembly.

(From here on, it is worth noting that I am 28 years old, 1.70m tall, about 110kg with an 8mm herniated disc and back issues.)

2. Chair posture, seat, back, waist, head, arms:
Generally, for about 7 years I have been working in IT companies, spending many hours sitting in office chairs. The experience this particular chair offers, I have never encountered in any office at any company.
The sitting posture at 90 degrees is exactly as it should be to relieve the lower back and back. The two lumbar pillows with springs follow you however you sit and always keep your waist in the correct position everywhere.

The back of the chair (above the two pillows) fits exactly on the upper part of the back while the back itself is also a pillow for the head.
Here, be careful: in a sitting position, you cannot rest your head on the back as when you lean back it is slightly raised and is not very comfortable (for me), so only in a resting position does it sit perfectly on the headrest.

The seat is amazing, it moves forward and backward like a car seat (about 4-5cm travel) and you can adjust it according to your needs and height. Here, it should be mentioned that when you recline, the seat reclines with you... that is, it follows the back so your tailbone doesn't hurt.

The chair's recline has a range from about 90 degrees to 150 degrees, feeling like 160 to 170 degrees. To understand what I mean, when you recline the chair to the maximum, it's like you're a bit more upright than when you sleep. The combination of the seat and the 2 lumbar pillows that follow you as if they're stuck to you is a very remarkable experience because in any position you lock the chair, it is always relieving with zero pain. Truly an incredible experience.

The arms are also attached and follow you in any position. You can do whatever you want with them, at any height you want, generally they are 4D so they do everything. Generally, I don't use them so they don't bother me and I don't bother them. Where I do use them is when I recline and hold my phone, and they are quite convenient.

3. The negatives:
The chair is exclusively for serious consumers who want to invest in their back and waist.

The arms look a bit fake and have a bit of wobble but that's not a reason to deduct stars from the chair.

The two pillows, because they are permanently attached to you, when you make a small lift to get a good position in the chair, they pull up your shirt and it bothers me a bit... obviously this is something personal which is not worth deducting stars from the chair, the solution is to dress like Obelix, wear sweatshirts and whatever else inside your pants and problem solved.

4. Conclusions:

I got the chair for 230 euros including shipping. The truth is that at first I was hesitant because I read the review of a member named @zeppos. I'm not saying that what the person mentions is not true, however, personally, I did not find many of these in the chair.
What I can definitely say is that overall the chair is value for money, there has been a study on body posture and this study has been applied 200% on this chair. It is incredibly comfortable and quite convenient. Finally, after about 8 years of sedentary work, I get up from the chair without thinking about getting up slowly so my back doesn't hurt, something which, personally, is priceless.

P.S. As I reread what I wrote, it was like I was advertising the chair. So, for the record, I was not paid for this review.

I was not impressed. At first glance, it looks heavy (22+ kg), solid and sturdy. It doesn't creak, it doesn't wobble. Assembly is very easy and doesn't take more than 15 minutes. Theoretically, it supports up to 150 kg. Various (usually paid) reviews claim that the mesh is durable and doesn't wear out after 1 year, a problem that is not uncommon in chairs of this category.

However, after 2 days of use, I just can't get used to it, it's quite uncomfortable because:
it presses on the back of the thighs, the seat is wide but also relatively "short" (that is, the legs stick out at least 5 cm more than I'm used to even if I pull the seat all the way away from the backrest), also in an upright position it is almost impossible to rest your head back unless you sit like a Buckingham Palace guard and have 0% kyphosis, the backrest only locks in 2 positions (one completely uncomfortable middle and one quite reclined that is only good for sleeping) and the armrests could be further apart from the chair, since when you rest your arms on them you feel cramped.

The lumbar support seems quite good and covers the entire lower back down to the glutes.

But the worst thing is that to lean back even a little (say 5 cm), the backrest is very stiff and you literally have to use force with your legs no matter how much you weigh... and you have to keep pushing for the backrest to stay in the reclined position. And it's not just me who finds the chair overall uncomfortable, but even my wife who is half my size (see: she passed it on to me and kept the old one).

At 200 euros, which is now the price of the black model (on discount, since the manufacturer sells it much more expensive), it might be a good option for someone who finds the chair comfortable. But the 250+ for the white model I almost regret. The 4 stars are completely generous, I will change them to 3 if after about a week I still can't get used to it.

update after 1 month of use: I've somewhat gotten used to it and will generously keep the 4 stars, but I've tried much more comfortable chairs in this price range. It is still uncomfortable and tiring for the arms (especially the ulna area) when sitting in a slightly reclined position, because the armrests are very hard. It is uncomfortable and narrow in the thigh axis when sitting completely upright, so you need to recline slightly backwards. Therefore, forget about locking the backrest in the upright position. The head rests properly on the back only when you recline slightly, but if you manage to find the right position then it is a pleasure (but this means that the computer screen must be raised higher than what I'm used to). Generally, I remain unimpressed and wonder why my review is getting 2/5 so far when no one mentions anything contrary to any of the above problems. Personally, I have found it comfortable only when watching something on YouTube, but not for work, browsing, gaming, etc.

update 2: on the purely technical features I agree with everything user Thanasis_Grammatikas wrote in his review (especially about the shirt lifting up when you move...), however I still find the chair marginally comfortable even now that I have gotten used to it. Yesterday I tried a chair at half the price at a well-known Swedish department store and honestly found it MUCH more comfortable and relaxing, at least in the 5-10 minutes I tested it. This doesn't mean it was overall better, because it doesn't have the same build quality, durability, etc., but from the Sihoo S100 I expected more at 200-250 euros.