In the past I've used stability balls in place of a desk chair to work my core but this might be more practical
We Tried It: Gaiam's Balance Ball Chair
Would I be putting a grain-feed cow on a fad diet if I took it out of the feedlot and put it on pasture eating the grass nature intended?
I've wondered about those. I'm sure it works fine as a chair, but it seems like having the ball resting on a base would cancel out a lot of the benefit of using the ball in place of a chair.
Vincit qui se vincit
Fitness and Injury Prevention - Somastruct