@Sygnus, i would have to agree to disagree, i have found in my experience they solve some of the common problems caused by people who like to fiddle with their computers like me, also as i do my own development i find it helps me to clear a program i create totally registry keys et all.
But i of course have no evidence it helps performance, just i guess as long as i know what i am getting rid of its not too bad

Though i use CCLeaner for the registry, its most important use to me is cleaning temp files and some of the other useful tools in their, such as the msconfig style tool; the fact that when a program refuses to uninstall i can do it manually etc