I got one of those powerballs, it's pretty kewl. The trouble with all these gadgets is pretty soon you forget to use them now and then...but I can recommend one of those.

BUT the NSD powerball is very good wrist exercise. You can also grip it by fingertips for finger exercise, but the wrist is where it works the most. I used it for a few weeks like almost every day for a while, and noticed I had to lenghten my wristwatch band!

I'd say it's good wrist exercise and also a good way to warm up your entire arm up to the shoulder, gives you mostly stamina to for instance keep picking rather fast for extended periods. If you could remember to use it daily I bet it'd give very good results, and I'm happy I got one if only for the novelty's sake, it's fun to show people and make them do some exercises or achieve some rpm level etc...it can REALLY feel very tough to keep it rotating after a minute or two.