I spray the surface with a small air compressor first to remove any dust/dirt particles, then do the water/microfiber cloth thing.
Our "RV" disc was really smudged with fingerprints upon purchase (yes it was new). I almost returned it, but got over my fear and went for it. It cleaned perfectly.
I have had tiny scratches appear on SD DVDs when cleaning this way, and I'm afraid to try on HD DVDs. Really love the BD protective coating.