I had one. It's a nice little toy, and is neat while watching TV or something, but if you are serious about exercising your wrist, there are better exercises you can do.
I have one, used it for a while, but those balls are object to wear more than one should like. Suddenly it gets more and more difficult to get the thing going. Plus, when you let others play with your 'ball' when it's already teared out a bit, it looks like it settled to their movement, therefore its more difficult again to get the thing going. This is not only the case with my gyro ball, several other people i know used it and they all worned out (the ball that is). Ah well, maybe it's brand dependent.