I've been using a 'light' Gripmaster (the blue one), as part of my self-directed rehab after damaging my left hand earlier this year. I should probably have held out for the 'extra light' yellow one to start with.

I have most of the strength back now, but I found that initially, there was too much counterforce. I think that recovering the fluidity of movement may have been delayed by struggling too hard at first. Having said that, I plan to move up to my old original, (now called 'medium') sometime next month.

I can recommend the Gripmaster as a way to develop the muscle groups you need for playing guitar. I warn against the temptation to start off with a stiff set. It should be fairly easy to compress. The idea is to train sets of muscles that usually don't get much use in any other activity.

One thing the instructions don't make very clear... do your exercises with your hand in the same sort of position it would be in while you are playing. You'll be amazed at how much more difficult it is with your elbow bent, and your hand up near your shoulder!