I bought a Powerball after having a go with my brother's.
He'd bought it for fun, but I found it such a good therapy I bought my own.
By day I'm an electrician, by night I play the trumpet / cornet and I use a computer a lot, hence over the years I've developed some mild repetitive strain injuries in my arms, especially the right fore-arm.
For years I've been just `putting up with it' as it's not usually more than a dull ache but if I've had a heavy day at work I notice my arm stiffening up, not good if I'm playing brass that evening.
I've found that if I use my Powerball when I get a moment a few times a day I can work some heat into the muscles which really loosens them up and gets rid if any aches.
I try and do about 10 minutes sustained at a moderate speed with short bursts up to full tilt, swapping arms as each one gets tired. Believe me, as the speed goes up, these things become really heavy!
What surprised me was the first time I went to band practice after using the Powerball just how much more nimble my fingers were in the running sections. I've always struggled in faster sections trying to get my fingers to go fast enough, but using the Powerball to loosen up beforehand makes quite a significant difference.
What's nice about the Pro model (apart from the funky flashing lights (mine's blue)) is the little trip computer on top so you can keep a track of how you're doing, as it can indicate speed, top speed reached, work done, and strength test, so you can keep tabs on how you're doing.
All in all, the Powerball is quite fun to use, in a similar way to a YoYo, but more importantly it really helps to condition your arm muscles. If you do anything in your daily life that involves using your arms with strength, speed or precision then you'll most probably find one of these a real help.