COD4 is one of the best games I've ever played and I recommend it to anyone. The best game of 2007.
During the game you play as both a British SAS operative and a U.S. marine. In the beginning you do a tutorial/mini-mission to prepare for for a mission to infiltrate a ship and secure a nuclear weapon. When you manage to escape the sinking ship thats when the story starts.
The story is fantastic and the gameplay is phenominal. Some say that the story is short but its not like 5 hours short, more like 10 hours short (which isn't great but I guess it could be shorter), but I don't mind because like Gamespot said its "of a very high quality." The game looks great, has solid voice acting, plays great and the control scheme feels great.
The A.I. on your team isn't of the stupid and unhelpful variety either, more like the helpful kind that can get you out out of really difficult spots but still allows you to do a lot so it isn't boring for you.
The games highest difficulty(veteran) gives the game extra intesity and makes you feel like your accually there because pretty much one or two shots kill you. Some of the missions on this difficulty I honestly can say I'll never beat. But also the online fantastic and probably better than Halo. In my opinion this game is fantastic but not perfect, it gets a 9.5.