
I have one....
They are pretty good, once you spin it up, it does tire your arm, so it must be doing something. It seems to exercise the hand/grip, wirst, arm and shoulder, which is quite good!
I have not followed the link above, but you can get them for varying prices, ones with rev counters/ flashy lights are quite nice, but the plain version is best if you just want the exercise. Get one with a rubber rand around it, as mine is just shiny plastic and can be hard to grip.
As and when you get one, when you are using it, try not to let it 'kick' -you will find out what this means - it damages its bearings and makes it rattle.
I have used mine in conjunction with one of those springly grip exercisers which has made my smaller fingers stronger and strengthened the tendons in my forearms.
Powerballs are slightly addictive. -Try spinning it up by hand, ie without the string.
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Mark