I received this power ball yesterday and after getting the technique sorted found it very addictive.
I was a little skeptical at first but after using it for a while could definitely feel the muscles in my forearm working. I now use it while watching tv which sometimes annoys others as it can get get quite loud when you spin it at a quicker speeds.
The only thing which i regret is not buying the pro version for £5 more as this has the Rpm counter which would be useful to see progress.
I would definitely recommend a power ball to anyone but i advise buying the pro version unless your not too bothered about how fast you can spin it...