I sincerely hope this doesn't turn into a debate again, but simply put, those who say CCleaner is worthless don't have the experience to really speak on the topic. I won't discuss other apps because I don't use them, but I have given several real examples of how CCleaner's registry clean solved issues for me, like failed ATI driver
installs, cleaning up after an app or driver removal that was preventing a reinstall, etc. There have been several issues that CCleaner has helped me out with. It's also free, and provides several other tools. It is far from useless.
It has never caused me a problem in years of usage on all of my systems. If you are worried, it provides a simple and easy backup method.