Great multiplayer, but struggles to hold up in single-player.
Story (6.0) :
As far as storylines go COD4 is poor to say the least. It feels incredibly disjointed and its very easy to lose track of what's going on. That said, it has very little impact on how you approach the game, other than determining the locations you go to, but don't expect to get emotionally involved with the characters or anything like that.
Graphics & sound (7.5) :
COD4 has received many accolades for its visual presentation, but honestly I can't see why. Its a very mixed bag. The character models and weapons are solid enough but the environmental textures are really poor at times. The set-pieces (including a nuclear blast) and the lighting are spectacular, but this certainly isn't the best looking game on the 360.
The sound design works very well, helping to make you feel like you're actually in a warzone. The musical score is also fantastic and is written by Harry Gregson-Williams, of Metal Gear Solid fame.
Campaign (7.0) :
The single-player missions in COD4 are very bland. They basically consist of walking forwards and shooting at people. This may seem like a strange criticism of a first-person-shooter, but games in this genre need to become more creative because the genre is so saturated.
There are one or two more interesting levels, including an on-rails shooting section and a stealth level, but these have become token elements of all FPS games. OK, in COD4 they're better than most in shooters, but the concept is nothing new. On the other hand, the game does a great job of creating the sense of being in a hectic battlefield, something which most FPS games really struggle with.
Its also a very short single-player campaign (you can finish it in around 6 hours). However, there are multiple difficulties and the achievements are only focussed on single player so you will come back to it more than once.
Multiplayer (9.0) :
Online COD4 is a much better package than most FPS games. The game-modes are very similar to other shooters but where it comes into its own is with its system of leveling up, RPG style. As you do well in matches you will earn new "perks" which you can choose from to give your character different abilities, such as "stopping power" (which increases damage dealt). This is a great addition which really lets you approach the online arena your own way.
You can also unlock a huge amount of customisable weapon options which further deepen the online experience. On top of all this, you have the ability to create your own character classes, choosing weapons and abilities. Overall, COD4's online experience is up there with Halo 3 as the most complete online console shooter.
Overall (7.5) :
COD4 is best suited to gamers looking for a great online experience. As a single-player game it does very little new, despite a couple of great set-piece moments.