I have to say, at first I thought these were a bit silly and not really worth the money. But after a friend brought one and I got the hang of it (which didn't take long) I wanted one!
These things a very addictive, and fun to play with as a desk toy. How ever on the day I got mine I must have been using it for about 3 or 4 hours and the next day I had trouble typing because of the pain in my wrists. Be warned this device is addictive, maybe too addictive!
Though it is nice to just play with in between typing, of which I do a lot. Its also great for climbers, because it really helps build grip, and gives you the advantage of more "contact" strengh on the rock.