Okay, so it's an older game, I know. However, I just got it (11 days ago), and thought I'd write a bit about it. This has to be one of the best FPS (First-Person Shooter) games I've played. The graphics are really nice, and the gameplay is nearly flawless (though I'd like a little more freedom in where I can go!). I'd recommend this to anyone who plays FPS games, easily.
In terms of single-player campaign, it's not quite as good as Call of Duty: Modern Warfare 2 (which I got on launch night), but that's to be expected. However, you eventually finish the campaign and you're thinking, "Now what?" That's when I switch to multiplayer.
Multiplayer. This is where it all goes down. Your skills, your strategies - everything is put to the test. One of the biggest reasons for that is how Call of Duty works in the first place. They don't use the Hitpoints system like older games such as Battlefield 1942. (It appears that the way they run it is becoming more of a standard - Battlefield Bad Company 2 (not sure about Battlefield Bad Company) also uses it. The more injured you get, the more red your vision gets. At a certain point, you're dead.
Anyway, this makes it a very difficult online experience, but it's really rewarding to see the diagonal "you're hitting him" marks with your crosshairs, and the little "+10" show up when you begin to get used to it, and can start rocking it. As a warning, something to be expected from any multiplayer shooter is that people tend to play for themselves. In other words they'll get in the way, and generally put less importance on the lives of their teammates, backing you up less (especially in Team Deathmatch mode).
The maps from the campaign (along with a few extras) are used in the multiplayer. They're well constructed, as well as fair (as in there's always a way someone could see you if they're in the right place - no matter where you are).
Overall, I'd give Call of Duty 4 - Modern Warfare a good 9.5/10. Excellent game.
Grynder