I bought a Powerball Pro about a week ago, as I have been having problems with my wrists (too much keyboard work).
An incredible little device which certainly seems to exercise the wrists and fore-arms. So far I am going fairly slowly (6000 rpm) as it quite quickly makes joints ache :-(
However, after a while I have found that the normal wrist and elbow aches are decreasing - so I am happy. Will post progress again after a few more weeks