One of the programs in my security suite, I forget if it's Spybot Search & Destroy or another one, pops a box whenever it encounters a registry cleaner during a scan: "It has never been established that cleaning the registry actually improves computer speed or performance. We do not recommend purchasing software that cannot be demonstrated to have any practical use." (Or similar words; I can't swear by the exact verbiage.)
For those who really want a registry cleaner, the techies at the shop where my system was built recommend Eusing Free Registry Cleaner. It's free, it does the job quite thoroughly, and it doesn't seem to mess up anything else, at least not that I've ever seen or heard of.