I had the InvisibleShield on my iPhone 3G, and ended up removing it to get a Crystal Shield and case.
The InvisibleShield surface would get REALLY icky, all smudgy and gross looking, probably at least partly from my hand lotion but it was pretty nasty. Cleaning the grime off required some sort of damp cloth, usually a glasses cleaning cloth.
I've never had an issue with the Crystal Shield getting nasty like that, any time it doesn't look perfect I can just wipe it off with my shirt.