I have been using Registry First Aid for some time now and find that it's excellent....
I've been using Registry First Aid since version 3.1. It's now on version 5.0.1. It has been excellent for me. I never had a crash and never had to roll any RegEdit backups. Well ... there was a System Restore once, but that was many years ago.
Registry First Aid won't fix any entries unless you approve them, so you stay in complete control. It also will rank the safety of each proposed fix Green, Yellow, or Red.
My only gripe is when I first ran it, there must have been over 1000 registry errors. It took me 2.5 afternoons to look at each one of those. Most can be fixed or deleted easily, but some require 3-5 minutes of research even when you know what you're doing.
It does provide quick registry-key links to RegEdit for doing the research as well as the file directory tree, but you still need to figure out what some installer (or uninstaller) couldn't figure out for itself. This takes study, even on a system that you are familiar with. I guess, if all the fixes were straight forward, you wouldn't need the registry in the first place.
The Platinum version will also compress the registry.
It also has a feature to search out obsolete software entries. For example, after uninstalling AOL, there are still 100s of AOL entries lurking in the registry. You can have it search out the string "AOL" to find those entries, then determine if they're safe to delete.
AOL is so inter-woven into the registry that tracking down all its references for deletion purposes is hard. I just don't have time to do it. But if someone has written a smart uninstaller that can track down latent AOL reg entries automatically, please let me know about it. I got latent entries for AOL client 7.0 & 9.0.
Cleaning and compressing the registry does make the computer operate somewhat faster. I've been meaning to get Microsoft's book on the registry, but haven't had time as yet.