These are well known for rapidly losing their tension. They are so cheap that it may not matter too much, and the top ones are plenty hard. But you're going to be hard pressed to know where you really stand strength wise with these as compared to others, if you care about that. Some people care a lot, others only care that they're stronger and don't give a damn about precisely how much and when.
Personally I really like the Baraban specialty narrow grippers(they make regular and narrow), in addition to the standard COC's. The broad range of gripper is easy to work on any gripper you get virtually no matter what size your hand is, but the narrow one lets you work that tighter range hard without spending any of your strength at the very broadest range. Since it's almost always the narrower grip ranges that people have trouble with and need to work most, you've got an ideal solution with a specialty narrow gripper.
There's a guy named Dale who is Baraban's distributor in the U.S.(Baraban is a German dude). He posts on
www.grippermania.com once in a while telling people he's putting in another batch order and asking if they want in. I got three from him myself, and he has a good rep. They're about 20 bucks or so, right around the price range of COC's.
P.S. I screwed up my thumb levering, so took quite some time off gripping. Be careful with those sledgehammers, boys! I used a bodybar, which is pretty much the same thing, to get pretty much the same type of injury. Happened in a quick second, and I'm usually super careful, so I guess it could happen to anybody.