I've been meaning to write this review for such a long time. I have had a Gaiam "Yoga Ball" Chair for probably 5 years now. The idea is that this chair helps to improve back and core strength because you have to be more focused on your body positioning to stay balanced on it. Except, you kind of don't...
The base of the chair keeps you so stable that I personally did not find it to be much work to stay balanced on the chair. What's more, once you are used to it you can still sit in positions that are really bad for your circulation (sitting on on leg, leaning awkwardly etc) Of course, that is user error. But I'm just saying..
After having the chair for a wile, the metal support you can see in the picture above broke off. This means at times, if the ball isn't pushed down into the base enough, if you sit on it too fast it could sort of pop out. lol. The back support piece can bend. Do not lean back on this chair. Sometimes that is a tendency because you can do that on a "normal" chair. Even if you shouldn't... But mine has a permanent dent that I am afraid will snap if one more person leans back on it resulting in a head-striking-the-floor situation which would be no bueno.
Basically, I sit on just the ball now. I still have the base for some reason. But I really just use the ball.
I personally would not purchase this chair again. It was close to $100 when I bought it and even though the price has gone down, I think if you are interested in this type of chair, you would be better off just buying a stability ball without the base. This way you actually have to work a little bit to stay balanced and you can easily roll around on the chair throughout the day to stretch and exercise your legs and core. Yes, the intended purpose of this whole concept can actually be achieved... for a lot cheaper too!