What is the sign of a good shoot-em-up? If you don't play online games then it has to be about singleplayer replayability. I have only returned to two of these types of games, the original Half Life is one, this is the other.
Having completed the game soon after it was released I had a nagging compulsion to reload the game onto my computer to play through the levels again. The first time was perhaps one of the best couple of days I have spent sitting at my desk. True, the game could be longer (and is if you buy the expansion), but that's not really the point. Instead of playing through similar levels, longer levels or more difficult levels where all you do is restore to the last save till you manage to get through the balance of the game is perfect.
The atmosphere and immersion you feel as you run around shooting the enemy is incredible. The graphics, though not up to HL2 or Doom 3 are more than adequate and the sound adds so much to the overall feel of the levels. AI is as good as any game today and, when you have one of your own grenades lobbed back at you by the enemy you will curse him personally. Getting shot by my own people for retreating in the Stalingrad level was also a high/low in my gaming life.
The beauty of this title is that you can be as brazen or as cowardly as you want (except for the above level). Health close to zero? Then hide and let your computer cotrolled colleagues ferret out the enemy till you can find a first aid kit. Feeling like Rambo? Go out and shoot the hell out of anything moving. Though you probably won't last very long.
I returned to this game the second time to relive the experience and also to make amends for hiding from the enemy like a big blouse in one of the levels letting my comrades take the bullets. Suffice to say, I made up for my cowardice this time.
Overall, I would wholeheartedly recommend Call of Duty to anyone who likes this type of game. You won't be disappointed with anything until you reach the end of the game.