I've started a new campaign in COD4 after finishing it around a week ago, but I can't play it anymore, it's boring, because it's so scripted to death. There's barely any variations. At least in Crysis I have a much better freedom of movement and decisions (how to approach certain places and situations), which despite the scripted form of the story line makes it a superior game in my opinion, and it'd still be superior with graphics entirely equal to those of COD4, simply due to the less-linear nature of it (and that's the main reason why S.T.A.L.K.E.R. is the game of 2007 for me).
But with that said, I must say now that the first time I played COD4, without knowing in advance what would happen, was an absolute blast, and it was very well conceived. Just like reading a book or watching a movie of a genre/type you like. I really liked it, I just don't see any interest now in playing it for a second time.