Any program involving REGISTRY editing is dangerous. Mess with it without a backup and the ability to restore that backup can be the end of your current install. I cringe anytime someone suggests using a REGISTRY cleaner. If there isn't something in particular that you're digging around for in there and you don't image your system so you can be sure of easy recovery I think it's not a good idea. That said, I've never had a problem with it at all. From what I have read, it's not an aggressive cleaning if you use the default settings. Make sure you backup before using it or at least do the backup when prompted during the process. I use the full version on XP. It has some more bells and whistles such as uninstalling programs and handling startup programs. I have used it for years and never had any sort of problem.
Any good IT professional knows how to use a registry cleaner. One does not just blindly let the program make changes without reviewing them. I have found that there are items in the registry that do affect performance and stability that are leftovers or remnants from either legitimate programs or rogue items that simply cannot be removed manually without a significant amount of effort and intelligence. The registry cleaners do the detective work locating them and then the human should review the reported 'problems'.