If you own a 360 and have Live you need to buy this game.
First off the single player has one of the strongest stories i have seen in awhile, even if it is a little short, and takes you to several varying and interesting locales. You play as both private Soap of the SAS and SGT. Jackson of Marine force recon, Soap's missions take place in Russia while Jackson's mission are located in the middle east. There is also one distinct difference between their two campaigns, Soap's missions are usually more covert and confined while Jacksons mission are usually much grander in scale. The story will keep you guessing as to what will happen next and has its fair share of twists and turns all the way until the end.
Now onto the multiplayer. COD4 has some of the best multiplayer I have seen in quite sometime, it's right up there with Halo 3 and Gears of War on this front. You start off only being able to choose from 5 premade classes but after just leveling up a few times you unlock the ability to create your own class, which is one of the best things about the MP in my opinion. With this unlocked you can make 5 classes of your own with their own weapons, equipment, and perks (special abilities such as the ability to carry more grenades, make your bullets more powerful, and blocking your signal on enemy radar). As you level up you will unlock new guns, weapon attachments, and perks to add to your characters. The MP fair ranges from your standard deathmatch to territories and headquarters game types. I do have one bit of advice for newcomers to the MP, check your corners, you never know when some psycho with a P90 is gonna be crouched behind a wall just waiting to unload into your ass.
All in all COD4 is one of the must have games that every Xbox Live subscriber should own.