Version 6 ('stable' build 1376) does not seem to work at all under Windows 7 x64. I haven't tried that build on Vista.
Recent nightly builds (I'm currently using 22.214.171.1240) are better; from my experience, Joel is right to claim that they are much more stable. There are still issues, but mostly these are issues of Windows errors which Eraser deals with less than elegantly.
Clearly, a nightly build cannot be as thoroughly tested as a service release (hence the honest health warning), but I have no reason to believe that the Eraser engine is not working as advertised in the newer builds.
From my attempts to run 5.8 under Vista, I'd hesitate to advise anyone to run it under any OS later than XP SP3; it runs into all the Windows security issues that Version 6 is progressively addressing, and so also comes with a health warning. In all of this, it is worth remembering that the main reason for moving to the new (and evolving) version of Eraser is that Microsoft moved the goalposts.
In circumstances under which no version of Eraser is at present as complete and stable under Vista and Windows 7 as 5.x was under XP, my advice would be to stick with 5.x for XP, but move to Version 6 (the new version, or a nightly build until it appears) for Vista and Windows 7.