It's better than cancer but not as good as sex.
It can be useful, but don't expect miracles. I view it as a handy one or two-click way to clean out Temp folders and Temporary Internet Files. I personally do use the built-in registry cleaner but only because it has only removed very low-level entries. I doubt that it has any significant impact that I notice, though. If I didn't always have a fresh Image of my C: drive I wouldn't take the chance.