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.