i use one called registry clean expert every once in awhile, because i constantly rearrange my start menu programs and install uninstall programs and after time end up with tons of obsolete registry entries, not sure if it really hinders performance or not but im a bit of an oc.
Yes, there good. They remove junk registry strings and can increase the performance of your system. Try CCleaner (free) or PC Tools Registry Mechanic (paid for).