It's hard to put down
COD4 keeps you coming back for more. The ranking system, the weapon unlocks and the challenges you get XP points for are all great and add to the experience without taking your focus from the core gameplay.
The single player game is great with some new twist in game modes and missions. The meat of COD4 is on-line. I think playing the story mode will prepare you for on-line. The replay value for COD4 is unlimited because of the great maps and modes you play on-line.
If you enjoy FPS games and are tired of the World War I, II theme games. This is a breath of fresh air.