Intense campaign + ridiculous multiplayer = best current gen FPS
There's probably not a lot that I can say here that you haven't read already. If you're on the fence about buying this game, and are looking to user reviews for help, then there's no need to read any further. This game should be in any shooter fan's collection.
If you're looking for in-depth descriptions of the campaign, the GameSpot and/or IGN reviews do it justice.
I ran through Call of Duty 4: Modern Warfare's single-player campaign in rather short order, and enjoyed almost every second of it. Inexplicably, I didn't touch the multiplayer portion until almost 2 months later. Actually, there is a good reason for that, and it's known as The Orange Box. At any rate, I eventually returned to COD4, and I've logged DAYS online with it since. The experience points you earn in the various modes increase your rank, which then unlocks weapons, camo, and perks (abilities such as holding more ammo, increased bullet damage, explosives, etc.). This pseudo-RPG leveling system fused with FPS gameplay that is equal parts frantic and tactical make for the most addictive multiplayer experience I have ever played.
Unfortunately, there are some issues. The lobby system works well, when it works. However, you will experience infrequent but annoying disconnections from games and lobbies. Host migration is essentially nonexistent, so when the gamer with the best connection leaves a game, everyone leaves with him/her. There's the occasional bout of lag, hit detection problems, and some people claim there is rampant glitching/cheating that utterly ruins their experience (I have not run into this personally, but the complaints are consistent enough that I cannot discount them).
There are also some additional annoyances that may bother you, such as campers and grenade spammers. However, this is more a problem endemic with many gamers in any online FPS. At any rate, it doesn't bother me enough to affect my score in any way.
COD4 is one of those rare titles that has lived up to the hype, and is well deserving of all the praise it has garnered.