I have one, and it really works well. Although I think if you get one you should get a good one, some of the cheaper models are.. cheap and might not work as efficient.
I have a NSD Powerball, really really well made.
http://www.powerballs.com/
I actually played with it a while ago (my fault), gonna spin her up again.
Edit: BUT I can assure you this thing will work a lot more efficient than a squash or tennis ball, after one run you're immediately too tired in the arms and wrist to get to the same speed goal again. Most of them also tracks the RPMS so you can actually track your strengthening progress which adds to the motivation.
I believe they will benefit in guitar, when it gets up to speed my fingers get tired as well, so something is happening.