I use it yes, however you will want to get a 64bit capable registry cleaner....personally I use Vista Manager for all my tweaking, and cleaning....highly recommend it. Its by Yamicsoft IIRC.
But yes I run CCleaner on Vistax64 and no problems to date with it.
There's most likely no real need to clean the registry so don't risk it with a non-64bit program. It's still safe to use the other cleaning tools in CCleaner.
ccleaner works except there might be 1 little bug in vista (or just vista x64...). Some registry keys cannot be deleted and keep showing up (mainly old winamp files after deletion)