ive always used it and never had any problems, dont even bother to back up the registry anymore
http://forum.piriform.com/?showtopic=25513
http://forum.piriform.com/?showtopic=45976
https://malwaretips.com/threads/is-it-safe-to-use-registry-cleaners.17270/#post-130157
http://www.makeuseof.com/tag/dont-believe-the-hype-registry-cleaners-dont-make-windows-faster/
http://www.howtogeek.com/113382/how-to-use-ccleaner-like-a-pro-9-tips-tricks/
It's not a one click solution for regular maintenance.