Curious, I didn't run any registry cleaner after my clean install of Windows Vista Ultimate x64. I've ran it for nearly a year in a now without any problems and I've installed and uninstalled plenty of applications during that time. So I have had 600+ extremely bad Registry errors from the start and no telling how many more throughout a year of patches, Service Packs and application installs/uninstalls. I must be *really* suffering. ;-)
I did a scan on my Windows XP Pro SP3 VM that I run in VirtualBox - using CCleaner, one of the most recommended of all registry cleaning programs. It found a handful of "issues" and nothing close to 600+. Most were missing shared DLLs from .NET Framework 1.0 (irrelevant unless you run apps requiring .NET Framework 1.0), unused file extensions (not going to improve performance), a few keys for programs that I uninstalled (not going to improve performance), ActiveX issues (irrelevant unless you run Internet Explorer), some open with and uninstaller references.
That's going to improve performance somehow? I promptly uninstalled CCleaner off of my VM. Why? Because it works without a problem now. No need to delete anything and risk causing more harm than good. If it's not broke, then don't fix it. ;-)
This is a good conversation, but I'll just wait till Geek comes up with some benchmarks...or maybe you can provide some instead?