Usually you get what you pay for ... a couple examples, one as recent as about an hour ago ...
A couple years ago I purchased a bunch of "value" clamps on eBay. You know the kind ... black with orange handles. Well, they stink. No clamping pressure, and if you try ... they break. I broke two this morning!
Then you have the mid-price ones. WC sells a good many of these every now and them (under the brand name Columbian, I think). They are fairly cheap ($6-$15ea) and work pretty good. I've been pleasantly suprised with them and would buy again.
Then you have the parallel k-body style clamps. Most notably, and most expensive, are the Bessey. They are top-notch, but still not perfect. That is what I own the majority of, but I have also purchased the copy-clone's from Woodline -- where they added a little innovation, but some of it actually makes things worse (still good, but Bessey wins that comparison in my book). Then you have the copy-clone from Jet -- now, those seem to be top of the heap. I haven't purchased any yet, but have used them a couple times in the stores and at the shows. More innovation, but in the right direction with a finger-trigger to release the clamp. Irwin has also now introduced a copy-clone -- but I don't have any information on those yet. They look decent though.
Clamps ... oh, clamps. If I could do it all again, I would avoid the cheapos ... more frustration than they are worth. I'd go for mid-line and top-line clamps ... and definately would have more Bessey-style clamps of ALL lengths.
Another thing I've been happy with are the Bow Caul Clamps. They allow you to need LESS clamps to accomplish the same thing. Very useful (and I'd like more of these too!)
http://www.bowclamp.com/