I use the ball for my turbojam workouts and it certainly works on your abs and other areas, once you get the hang of it. Make sure you take things slow at first until your body adjusts to the balance. Make sure you also inflate the ball to the appropriate level. When you sit on the ball, your legs should be at a 90 degree angle at the hips and knees.

I've just started using it and not consistently yet so I can't speak as to its effectiveness but I think as long as you start out slow and accustom yourself to the ball, you'll be okay. One thing I realized is that you can inflate it more than you think you can. You can see if it's going down just a little too much when you're on it. I was still able to pump more air into it to make it stronger. I followed some of Leslie Sansone's tips to get used to the ball. I sat on it and alternated kicking my legs out. From that position, I then slid down the ball til my back was resting on it and basically lifted my my lower torso/butt up and down. I've lain on it sideways to do leg lifts, I've lain on it chest down and done push-ups (my own little fun thing to be kind of levitating for a while, lol). Oh, keep your sneakers on, better traction.

You might also try finding either Kathy Smith's ball workout (can't think of the title but it was from a few years ago) or Karen Voight's BLT on the Ball workout. They both have lots of pointers -- you might even be able to find them at your library.