I've tinkered with a few of these over the years. Most of them want to remove "stuff" that I use all the time, so they get heaved out PDQ. The first time I used one, it zapped some number of files, then asked for money to complete the job.
I don't bother any more. From my experience, dross in the registry doesn't seem to make a noticeable difference to the speed of the machine.
What is useful is Regedit, for removing nasties or stuff that won't remove in a more civilised way. Great caution is required (and a backup before you start, so you can "undo" if it all goes pear shaped).