Bob is right. The "Grip-Tite" knock-offs are pretty poor. I was referring to the clutch style bar clamps. Take care here as well as some of the castings are no where near square on the face. Its the typical HF diamond in the rough search.
I assume everyone probably picked up the Irwin deal at Lowes and elsewhere; 4, 6" Grip-Tites for $20. I think Irwin has these priced high at the normal price but, for $5 each these guys come in handy.