I bought a set of the the Stanley 2x4 clamps a couple of years ago. The yellow bracket and the screws are metal, the rest of the black part is plastic, but they seem fairly rugged. That being said, I now have several pipe clamps and uniclamps and so the Stanley clamps are never used.
As was mentioned in the comments somewhere, the challenge is that 2x4s are pretty much useless as serious clamping bars, they are usually twisted or cupped in some way and are often too soft to actually hold up when significant force is applied. So what can happen with the Stanley clamps is that they will dig in to the 2x4 and therefore no longer be parallel to each other.
So in summary, product itself is OK quality but the dependency on 2x4 for bars makes other designs far superior in actual application.