My brother has hand grips and I used them when he still lived at home. I feel it makes my hands a bit looser after I used them. Maybe people use them because they have a lot of stress in their hands and want to loosen them up before playing?
I used the Pro Hands Gripmaster when I was starting to play the bass. I stopped learning when I started to focus more on hockey, but that was a good precursor to building wrist/hand/forearm strength. I just use a light weight model...but I'm sure I could move up.
similar to noobman above, started using them to rebuild strength after busting up some bones in my hand.
Very addictive - ended up getting some of these suckers: http://www.heavygrips.com/ that go up to 400 lbs. Working on 250 at the moment. Bones in the hand still click every now and then when using them.. wonder if that's bad..