Firstly, the extra maps "included" with this still have to be physically downloaded (but free) so don't expect to just whack it in and get on with them. They are quite good maps, and make a change from the usual map listings once you've been playing the game for a total of 6 days (144 hours)!
Right, on with the main game. The most stunning thing about this game that you immediately notice is the graphics, they are absolutely unbelieveable and look gorgeous, even on a non-HD tv. On HD, it looks even better. Even when turning your head/camera, the graphics stay as smooth/crisp as ever. The controls are also sublime and are very intuitive.
The single player element to this game is cracking and is an excellent storyline. The levels for each of the missions are brilliantly designed, and offer just the right difficulty curve to keep you hooked and to consistently challenge you. It is great fun, and is definitely worth a couple of replays at higher difficulty levels to get the full experience. The only issue with the single player, as has been mentioned several times before, is that it doesn't last very long. You could easily do the single player campaign in a day. If anything, I think this is a testament to the enjoyability of the single player missions that you don't want to put it down, and would rather carry on playing it till the end!
BUT, multiplayer is where this game is at. Currently, CoD4 is the most played game on Xbox live, and with good reason. It takes all the brilliant graphics, level design and weapons handling and chucks them into an extremely user friendly lobby/server system, which will have you playing in a minute at most. On most of the game modes, you can respawn within a second, which is great when you just want to get on with the fight. Probably the best feature to CoD4 (and FPS as a whole) is the Killcam. This gives you a quick 5 second replay of your death, from your killer's point of view, and is extremely useful in exposing your flaws, things you can improve etc. The other great feature is the ranking system, and the perks/weapons you can unlock as you build up experience. Once you get to the highest level, you can even reset your rank (this is called prestige)and start again with the basic weapons to work your way up again, but with a unique rank badge for that level of prestige (10 in total). This only resets your rank and weapon availability, your total kills/score is still kept on the leaderboard.
The only downside to CoD4 is that it doesn't have the Rainbow Six Vegas feature of cover behind a wall (and peeking around/blind fire) if it had that, this game would be 100% perfect.
If you haven't bought this game by now, what are you doing? Get involved ASAP, before they release CoD5 and everyone jumps ship to that!