Brandon,
Sinclair has actually produced a couple different models of their concentricity gauge. There's several others out there (NECO, RCBS, Bonanza, etc.) as well, and I've probably used at least a dozen over the years. This includes a few made by custom smiths/machinists to my specifications.
I like the Sinclair better than anything else I've seen, including those I've had made to my specs/design. It's the one that still sits on the top of my reloading bench, as opposed to buried in one of the drawers beneath. Looks like I'm not alone in that opinion based on the other comments in the thread.
Kevin Thomas