A friend gave me this exact same powerball as a gift, i have used it every single day for about 2 months and the results are incredible. My forearms, sholders, biceps and triceps have toned and grown solid and they are continuing to grow.
At first i thought i could only do your forearms which makes sense of the wrist rotation, only found out a few days ago that this powerball can exercise more than just your forearms, it can also do the pecks which is what i am going to use it for most of the time.
The string that comes with it isn't really needed after about a day or two of use when you can learn to start it with your thumb or knee but learning to do that gave me a blister lol.
I will probably buy another one for ambidextrous use or a different model when this one gets old, the only bad thing about it is the noise can be annoying to some people when you get it in the fast speeds but i use it alone for that reason and its worth it for the muscle gain from it.
This product is excellent for someone who wants to build muscle in most of their upper body and arms, also people who play any sports to increase strenghth.
Worth The Money!