One of the things I really like about my KL Audio ultrasonic machine is that the only "cleaning fluid" it uses is distilled water. No gunk, no chemicals, no minerals, just clean (and dry!) records. I also have a VPI 16.5 that I use first if a used record looks really dirty. I've never had either machine make a record "worse".
There have been times where I've wondered how much they really do, but a lot of times a record is just pressed badly, or worn and no amount of cleaning will help. It can be very effective as well. I remember a time I had my girlfriend over listening to music with me and the record we were listening to was really noisy. She said "why don't you put that in your cleaning machine" to which I replied, "it's probably just worn out and I doubt that will help", but after a good cleaning it was noticeably better.