No personal experience but my brother swears by them. He had a disc in the middle of a Babylon 5 set that he somehow "ruined" and it plays now. I wish I had more details to give but I don't.
My understanding is that there are two types. One removes layers of plastic so scratches become shallower and shallow ones are removed. This works well but can only be done once and makes the disc more fragile to the data level. I guess it's like buffing a scratch off a car.
The other uses clear wax to fill in the scratch. I've tried this on cd's and had little success. To the naked eye the disc looked better but it didn't seem to play better. I've never had an unplayable cd though, or at least one my walkman with it's anti-skip can read through.
My local blockbuster used to use one regularly that was, I think, in the second category. It was a spin design that put wax on evenly. As them and maybe at a gamestop. Places like that probably have a lot of experience with damaged discs.
Nun sacciu, nun vidi, nun ceru e si ceru durmiv.
I know nothing, I see nothing, I wasn't there,
and if I was there, I was asleep.