I've used it for a couple of years now, and there's not even a hint of instability. It doesn't creak, flex, rattle or anything. Maybe because I'm not fat - haha! It is very comfortable to sit on, and the foot support is in the right place. It's also very lightweight.
So high marks to Thomann for this Guitar stool. (I assume it's Thomanns own brand). I didn't find much use for the integrated guitar stand, so I took it off.
The one thing I would change would be to make it a little higher. It's a little low...
And if you could fold it into a smaller footprint, that would be nice. But I guess that's a different kind of stool.
An added bonus is that you can use it around the house. I use it in the kitchen when I'm cooking complicated stuff....