So good it spoils you...
To save you some time, the sound, music, graphics, gameplay, AI, and map design are all absolutely top-notch and everything a FPS gamer could possibly want. I'm not one to throw perfect 10's around, so we'll give the sum of the above mentioned parts a 9.9. Yes, that good. And yes, better than anything Halo has ever done.
The main knock about COD4 is the "short" single-player campaign. I'm not saying it's long, because it's not. But I wouldn't call it short, either, and I surely wouldn't ding the game because of the single-player campaign. Basically the length of the single-player mode is determined by the player's style and selected difficulty level. You can choose to fly through it if you wish, or you could choose to ramp up the difficulty and really fight through the game. Up to you, but I'm calling bogus on the "shortness" of the single-player campaign.
So to sum it up, if you're thinking about buying this game. Stop thinking. Go buy it like now, like this very second. Stop reading this now and go buy the game. NOW! Why are you still reading? You're wasting time!!! Ok that's it:) Hope you enjoyed.