A barbell is a hell of a lot more useful. With a good barbell, you can do finger curls, which are essentially similar to the motion of the gripmaster. Plus the barbell is used for all kinds of other lifts.
You could use dumbells instead if you wanted.
Also, get a big freaking hammer and do wrist rotations with it. Hammers are useful for all kinds of other stuff too, so again, money well spent.
Get a big wooden dowel, drill a hole in the center, and thread a...