Increasing forearm strength/ grip strength

Anyone used a powerball before? Had a chance to play with my fren's powerball today, seems quite good for building up forearm strength. It's non-impact compared to doing wrist curls, which I find is easy to over-strain.

Now my question is, is this actually useful for golf? One of the claims of the powerball is that it can help to increase driving distance (but then again, all golf-related manufacturer make the same claim to sell their products)

Any strengthening exercise sure to help but Methinks it is more to work on the core muscles i.e trunk, abs and lower back muscles. Oh, and at the same time, to work on flexibility of the muscles, especially the big back muscles like the lat dorsi.