[masraum]

I've had and used a couple. I like them somewhat. They do help with your posture and your back and shoulders feel better when you get out of one.

The problem that I had with the two that I had (one was very similar to the last pic above) was that the quality wasn't there. The one similar to the last above had the underlying board of the seating area crack and the foam wore out what I considered to be prematurely which made my knees sore. The second one also had crappy foam that made my knees and butt sore, and was also covered in a material that even when I was wearing jeans or dockers, made my knees feel weird.

Basically, I like the seating position, but didn't like the chairs themselves.

I think you can get a similar feel from a good adjustable office chair. One key it to keep the seat bottom angled the opposite direction from the norm. Keep it angled to make you slide out, not lean back.