I have used an exercise ball instead of a chair for the past year or so and really like it. I tend to have a lot of nervous energy, and being able to bounce on the ball while still seated at my desk help a lot, without having to get up, walk around, and getting too distracted from what I am doing. A couple of caveats, though: 1) You really do look pretty silly, especially if you bounce 2) Some people develop lower back problems when using a ball for long periods; be careful! 3) Office mates always seem to like stealing it and kicking it down the hallway (which is actually a lot of fun, but also annoying) and 3) If you had beans for dinner, beware, the ball tends to amplify sounds from down there :)