My mom got a big purple ball that I love--I enjoy being able to bounce, and it just makes me feel happy looking at it. It looks like a big toy and I like that =]
But we've had it for almost a year, and we always use it to sit on at the computer or while I'm doing homework at the living room desk. Mom originally got it for "tightening of the abs and core muscles as you sit comfortably at your desk" but none of us had noticed a difference, physically.
So it's cool/fun, my back doesn't hurt when I use it (sometimes mom complains though, but she's like 59..), but it won't make you lose weight or give you abs or anything, in our experience.
I've never heard of those kneeling chairs, though. They look really interesting, but I don't see how it would benefit in any way... There are some chairs on that website that I would personally buy just because they look cool =]... I'd go for it.