Does it really work? I.e. Can you build your arm muscle with this?
You could build strength but not size no matter what anyone says.
After time you will build muscle with a powerball,It will exercise and strengthen your muscles from the tips of your fingers to your shoulder and up into your neck. Depending on how you use it, you will exercise different muscle groups.
If you want to build muscle go to the gym
If you haven`t got time for the gym have a look at this link
http://www.trainforstrength.com/exercises.shtml