If you have the means, go out and buy this game.
NOW.
I cannot recommend this game highly enough. Seriously, it is one of the most amazing, intense, enjoyable games I’ve ever played. And I say “enjoyable” instead of “fun” for a reason. I had a great time playing the game, but it’s hard to call one of the most accurate depictions of combat in any medium “fun.”
The multiplayer aspect on the other hand is one of the better team based FPSs I’ve played in a while, but more on that later.
Call of Duty 4: Modern Warfare, available on PS3, Xbox 360, and PC (I played it on PC), was developed by Infinity Ward, the developers of the first two Call of Duty games. Other installments in the series (Call of Duty 2: Big Red One, Call of Duty 3, etc.) were developed by other teams at Activision to a lesser degree of success. Not bad games, just not up to the standard of the “real” Call of Duty series.
Anyway, back to the game itself.

The game, as the name implies, attempts to give players some kind of idea what modern warfare is like. It’s a lot less two-hundred guys charging a hill and more five guys trying to find cover in the middle of a firefight. Without giving too much away, you play through related conflicts as two different soldiers, one a member of the British S.A.S. special forces, and the other a U.S. Marine. Although not based exactly on any specific events, the Marine’s missions take place in a Middle Eastern country and deal with the U.S. moving in to overthrow a dictator. It’s not exactly hard to see what they were going for. The S.A.S. soldier’s missions mainly take place in the former Soviet Bloc, and foreshadow future conflicts that could arise due to political instability in the region.

The missions are strung together by briefings and news broadcasts, usually of events that have already happened in the game, displaying how military operations, especially covert ones, aren’t always show exactly as the happened, even if the general idea is right.

Anyway, this game is intense. Even on the easiest difficulty you end up seeing the tastefully done, and sometimes thought provoking, death screens quite often. They just have the screen fade to gray and display either a quote about war, like “Nothing in life is so exhilarating as to be shot at without result.” -Winston Churchill, or some interesting fact, like that a single B-2 Bomber costs $2.2 billion. It’s much better than just some tacked on “GAME OVER” screen, and helps add context to your actions. Higher difficulties make the game even more difficult, increasing realism. As in, one shot and you’re dead realism.

Beating the single player game also unlocks an Arcade mode, with a running score and set lives for each mission, so that adds a little replay value, but the multiplayer really is good enough to stand on its own.

Call of Duty 4’s multiplayer is amazing. Great weapons, good balance between side for team games (on most maps anyway) and RPG elements to keep it interesting for a long time. By getting kills, capturing points, or completing other tasks you gain experience which progresses you through the ranks, unlocking new weapons and abilities called perks. Between the different weapon layouts and perks, you can customize your character to fit your play style.

Again, I cannot recommend this game highly enough. Whatever system you play it on, it is an amazing experience and should not be missed by any gamer.