Amazing!
First the graphics are superb and especially in HD. The details in the environments is brilliant and really brings the game to life. The gameplay is intense with fast paced action mixed with stealth based missions. The pace can be difficult at the start especially on multiplayer but you quickly get used to and realise how slow other games are! The story itself is again intense and rather alot of twists and turns occur as you progress.
Finally the multiplayer is again the best FPS multiplayer to date. The ranking system really promotes lengthy gaming streaks to unlock various weapons and modes. The only downside to the multiplayer is that at the start you have fairly poor guns and get beat quite alot by the higher ranking players. However once the early stages are over it quickly becomes addictive as you get better weapons and attachments.
Overall I would say that CoD4 is the best FPS on the market and although the single player campaign is short it is brilliant and the multiplayer is amazing.