The ball takes some getting used to, but I never want to go back to a regular chair.
Now, granted, it will never be as comfortable as a nice big padded, form fitted chair, but I noticed myself constantly slouching and leaning back and getting in as close to a laying down position as possible in my old chair. I didn't like this because it made me feel stagnant and tired. It is perfect for those who have to be or find themselves in front of the computer for long periods of time and want to get some exercise in.
The ball forces you to do a few things:
1. keep your balance and stay more in tune with your center of gravity
2. use your midsection to keep you seated
3. keep your back straight and have better posture
On top of that, there are a ton of exercises to do on the ball, so when I'm bored or have nothing to do, I'll just roll back and do some crunches or whatever on the ball, then roll back up and continue to use the computer. Sometimes it's fun to just play some iTunes and bounce around on it, it's just added exercise during times when I would normally not be getting any.
Search google, there's tons of sites talking about how good it is (health wise), sorry for being so long winded, but I really enjoy the ball.