It periodically finds and deletes some unused registry entires, but it is miles from actually cleaning out the accumulated detritus in the registry. I would say for every 100 obsolete keys and values it finds one.
I do believe this has been addressed in the latest 2004 version. I also run it regularly and see the registry errors that it is correcting. As I mentioned above, it found and corrected 497 registry errors on the PC that I recently brought back to life. ;)
I forgot to mention the One Button Checkup. One click and you can run the most common utility routines, neat little feature.