Id reccomend the COC grippers as were mentioned here. Theyre very nice. I have 3 pairs of them, the trainer, number one and number two. As for reps, I like to take them to failure, as I do with most lifts. Depends on your goals really though. I just do them in my spare time outside of the gym. Playing with them I was able to close the #2 in a short period of time. I think its better to hit failure in the 20's then repping out 100. I was getting over a 100 reps with strong store bought hand grippers so I went for the heavier ones. My crushing grip got alot stronger as a result. I found that when I could hit 23 reps with one gripper, I was able to move on to the next. As was reccomended on the package. Atleast this was true with the #1 gripper. The trainer I could get over 50 times shortly after getting it.