I have been a user of CCleaner (and Revo Uninstaller) for ages, and, coincidentally only today I was using CCL experimenting with it to see just how much disk space it could
recover if I set it to be be more aggressive than usual - e.g., including (for the first time) getting it to zap the Font Cache. The results were quite impressive.
I think @hpearce
is correct though in what he says. Anyway, I won't be getting CCL to zap the Font Cache very often.
One problem I have noticed though is that using registry cleaners seems to occasionally blow away my xplorer² bookmarks (they are held in the Registry) - which is very frustrating (tedious to rebuild my bookmarks). I have now set up a regular backup and easy restore process for the bookmarks - on the advice of the author of xplorer² at http://zabkat.com/
Incidentally, he reckons that registry cleaners generally are a waste of time, but I can't agree with that as it does seem pretty certain that junk tends to accumulate in the Registry, over time.