I use this now to warm up before doing any weapons work or before working on my proper grippers (Vulcan adjustable grippers from David Horne) and Heavy Grips 250+300. As such these only become a challenge when I've done LOTS of reps (50-150+).
What I really like about them however is, once I've done that many reps, each finger feels seperately warmed up and my typing/piano playing feels easier and quicker. Never mind that I get enough blood through my hands to avoid injury on the heavy grippers.
If you weigh over 250lbs (as I do) and can chin up that weight properly even once, then you probably will find these a little too easy, too soon. As a pure strength aid alone, not for the bigger guys. Having said that, even for someone like me, I would buy again if I had to. Great warm up tool for at least 4 different things I do with my hands! :P.
I also understand that rock climbers etc don't need crushing type grip exactly, but an enduring grip-so lots of reps on each finger would make far more sense.