These are some great grippers! I have the #1, #2, and am working on the #3 right now brutal workouts. Another thing to help with grip is levering also called sledge hammer training. I had an old barbell that I wasn't using anymore so cut it into 24 inch piece and the other was I think 36 inches or lil bit longer, with that I clamp on a 5lb weight or more to the end and do various exercises (holding bar at 1 end and lifting up/down like a handshake, helicopters, rotating side to side etc)
Personally I found that just training the grippers few times a week I wasn't making great gains once I started doing the levering my grip increased alot. Oh and if you get a chance try to watch Magnus Samuelsson The worlds strongest arms dvd, shows various clips of how he trains and everything that man is a beast !!
oh forgot to add he was also in Sweden's Dancing with the stars
Magnus Dancing with the stars