FWIW, I can attest that the OneCare registry cleaner removed keys which then
prevented the installation of critical security patches for Office 2003.
So, nobody in this business is perfect.
My own recommendation is not to use these products except in unusual
circumstances. I heard from one user who is a developer who feels that he
gets good results because he regularly tests installation and removal
routines for his software, and the registry fills with junk. I'm willing to
believe that a registry cleaner works for him, but I am not willing to
recommend them for regular use by the average person, despite Microsoft's
OneCare offering. I would assume that the issue I ran into was fixed
thereafter--but as you can see, it is still vivid in my recollection.
