To expand on what Matt said; get a second guitar, one with high action, larger neck, and heavier strings. Play it (instead of your regular guitar) for 1/2 hour daily for a couple of months, you will be amazed at the difference it will make in hand strength and endurance. When you switch back to your regular guitar you may even want to change to heavier strings, (I did, I was overbending everything.) If you want to improve both strength and stretch, by all means play bass for a while, the longer neck and bigger strings will definately give you a better "work out" than any of the gadgets. Also, none of the exercise gadgets will improve your dexterity or playing skill, only practice can do that!