Yah I sell and use the Monster Screen cleen product as well. I have been using it for quite some time with the electronics i install and it weems to work great on the phone. Most of the time all you need to use it the cloth that it comes with and not the spray.I'm sure Windex wouldn't damage it, but like its been said, I wouldn't try it either.
I used to have a cleaner called iClean by Monster. It was great for cleaning my entire Macbook as well as my ipod and I used the little i had left when I first got my iPhone without any problems or anything.
I now just use a very lightly wet lint free cloth (the one that came with the cleaner I got is perfect size) and clean it that way without any problems. It gets all the fingerprints off and stuff. I also recommend the entire front protector from a company called BoxWave.
I had bought their iPhone screen protector and 2 days later they came out with one that covers the entire front, screen top and bottom (except the actual home button). They sent me a nice email and said they would ship me the new one at no cost to me which was a nice gesture. Its very cheap and is reusable.
Microfiber cloths are your friendForget all the above! Get a eyeglass cleaning cloth at any eyeglass store. You won't regret it. It removes ALL fingerprints and leaves that screen as shiney as the day you first opened the box. I always hate to touch the screen after I clean mine! Best money you will ever spend!