I have used two. I had a generic one in my old office and I have a variable balans in my home office. Kneeling chairs are fantastic. You really feel a lot more solid from hips to head after sitting in one all day, and less like a wet noodle.
The main key to getting the most out of a kneeling chair is to make sure your monitor is at the right height. Kneeling chairs make you sit taller and you will run into neck pain if your monitor is too low.
Kneelers really highlight that mainstream ergonomics is trying to sell you the wrong thing with the latest, greatest lumbar support. Fixing your hip angle automatically restores the curve in your lower back.
