I suspect most readers here knew, or at least suspected it all along: most registry cleaners are, at best, useless.

Lifehacker has a good piece on this, although it unfortunately begins by talking about how to edit the registry, which is really something I’m content with people not knowing. The people who have good reason to edit their registry know it well; I’ve never, ever heard of a non-technical user having reason to edit anything in the registry.

In bullet points:

• Messing with the registry is dangerous stuff. So is “cleaning up” DLL files, which most registry cleaners somehow feel compelled to do, too.
• Unless a particular entry in the registry is actively breaking something, deleting a handful of old registry entries isn’t going to help performance or disk space when there are hundreds of thousands of them.
• Defragging the registry is silly, since the whole thing is stored in RAM.
• No one has ever produced statistics showing that cleaning the registry has led to any appreciable increase in speed.

I tend to think that registry cleaners are like colon cleanse products: aggressively marketed, pretty much snake oil, impressive-sounding without giving any hard statistics, and really freaking dangerous.