After spending A LOT more time with the game over this weekend, COD4 is definitely a candidate to me for GOTY and is easily the best FPS at this moment surpassing both Halo 3 and TF2.
People have complained that the single player was too short, around 6 hours is what a lot of reviewers were saying, but to me, it was the perfect length. I personally can't see anybody beating it in 6 hours unless they are playing it on Easy and somehow avoids dying at all.
The Multi-player is definitely where this game shines. It's simply fantastic. The inclusion of RPG elements adds another layer of depth to an already deep multi-player experience. All that's missing is the matchmaking quality found in Halo 3.
My only gripes with the game, besides the matchmaking being not as good as Halo 3, are issues with the single player campaign namely the infinite spawning enemies, until you trigger the next set of events, and secondly, the enemies ability to throw grenades with sick accuracy.
That said, Call of Duty 4 is a fantastic game.