I now have both a Herpstat ND and a regular Herpstat. They hold temps very well, within a degree. The ND is on the 4 Aneries, and the standard HS is just on Butter. Vern's viv is running a 500R and I might change that out to a standard HS after the holidays.
I'm not ditching the 500R, though, and might even by another one or two. I saw a great idea to use the 500R as a 'fail-safe' behind the progressive stats. You set up the 500R first, and have it set to kick off at a temp a few degrees higher than you want, but still low enough to be safe, say 88*. Then you plug the progressive stat into that, and set it to the temperature you want, like 84* or whatever. That way, if the HS fails in power on mode, you won't get run-away temps. BUT, that also means you need to dial it up now and then to check that the 500R is still working.