Lee,
I have been looking at this type of clamp as well. We have a set at work for our Bridgeport. The only problem I have seen is that the steps on our set are slightly angled in towards the body of the part. I presume this is to stop it from sliding apart when used with a bar. I was thinking about having a go at milling one with a section of bar clamped to the table at a slight angle.

Ian