An amazing game for both single player and multiplayer, with the only fault being it's short length.
The characters each have different personalities, which is nice. It makes you actually feel like a part of the squad.
As far as the game play goes, this is game is nothing short of amazing. .50 caliber sniper rifle missions? Yes. Stealth missions? Yes. Helicopter gunner missions? Yes. It's all there, and the game play is flawless in all of the scenarios you're placed in.
The only thing that keeps this game from a 10 is it's short campaign. You'll soon forget about that once you start multiplayer, though.
Multiplayer over Xbox Live is great fun. You can form clans (clan tags are available once you get a higher ranking), play different game modes (although people usually tend to stick with the deathmatch modes), and customize your character. This is possibly the best aspect of Call of Duty 4's multiplayer. You can choose what traits your character has (do you want him to have more health, do you want your bullets to do more damage, it's all available), what weapons you use, what add-ons you have on your weapons, and even what your weapon looks like. It really makes the game a nice change from other online shooters.
In closing, this game is well worth the purchase, especially if you have Xbox Live.