I had been mulling over getting one of these for ages. I had a bad wrist injury about 6 years ago and since then I have had weakness in my wrist/arm. I had all the usuall treatments from physio but mostly boring exercises was the end result and because of my job it was hard to get in any regular excercise.
Now with the powerball it takes the boredom out of excersising. I can definately feel it working my wrist in all the same places as the exercises the physio gave me but with the bonus of my arms also getting a bit of a workout as well.
That coupled with the fact I can do this at work makes this ideal for me and by far one of the best products I have bought in years. Only thing is its going to take a while to build up the rythem in my left hand as I am all over the place and find it hard to keep it going but it is getting there.
BTW. If you are looking for replacement start cords try a shoe shop. The laces on my daughters shoes are identical to the cords with the exception you may have to cut the small plastic sheeth at the end down to the same length. I bought myself 10 pairs for £1.50 so now have 20 replacement cords (and they do work).