There seem to be several manufacturers of this type of clamp (or one manufacturer with varying quality levels) judging from my selection built up over a few years. The best of them are OK, and they go downhill from there.
If you must buy some try to see them before you do,
check the quality of the castings, make sure that they are clean,
check that the securing mechanism of the pin that holds the head to the bar doesn't fall apart when you sneeze on it,
check that the screw operates freely throughout its range,
check that the securing mechanism of the screw to the moving part of the head (either a grub screw or a small pin) doesn't fall apart when you sneeze on it,
check that the tailpiece moves freely along the entire length, and
check that the tailpiece locking mechanism engages fully and freely with the cutouts in the bar along the full length.
If all that's OK go ahead and buy, otherwise look elsewhere. If you can find good ones they are a cheap way of building up a useful collection of clamps, if you buy bad ones they are a cheap way of raising your blood pressure even higher than is usual during a glue up. Oh yes, and don't expect them to force assemble tight joints (not that you or I would ever do that, eh?
), as noted earlier the bars are soft aluminium and will bend/tear under undue stress.