Call of Duty 4: Modern Warfare is a great game. A little overrated, yes, but fantastic nonetheless. It plays like an absolute dream, though the controls aren't quite as perfected as Halo 3's control scheme (this is arguable).
The game is linear, meaning there is only one path throughout the game. However, this isn't a major problem as the game is always intense, and there are plenty of large battlefields with objects scattered all over for cover. The game always feels realistic and never delves into the realm of fantasy, which is something I like. I also like that the series is differentiating itself from the gigantic behemoth that is the subgenre of WWII shooter. The series has always stood out in the swamp of crappy WWII shooters, but now in its quest to further carve out its own niche, it really stands out as one of THE premiere FPS series on the market.
Granted, it is linear, and it rarely strays from its near-perfect formula, but this game wins you over with in-your-face realism and intensity. Games like the piss-poor Hour of Victory made all the sounds of war--explosions, gunfire, shouts--but it never actually put that in your face. It was always out of your reach; you were never actually there. In CoD 4, this isn't so. All of that is going on, but it's always right next to you, and to avoid getting hit by all the bullets and explosions, you have to constantly move, and view the battlefield with a tactical eye. You must always be in cover, and when you leave it, you have to know where you're going to go--standing around in open view of the amazingly intelligent enemies will get you shot down in no time.
This is why the linearity doesn't matter: It seems like you're in a real war anyway. This isn't over tactical, like the mediocre GRAW series (which people may disagree with me on), but it's damn realistic, and pretty freaky. I can safely say that this game is as close to war as you'll want to be.
The controls are pretty straightforward FPS action, and they work amazingly as I said before, so they're not really in need of much explanation. The graphics are simply jaw-dropping, however. The look of this game is damn near flawless for this generation of games, and while not quite as awe-inspiring as Bioshock, they are fantastic. The texturing is perfect, and I think the character models are among the best, though not quite as good as, say, Kane and Lynch: Dead Men (that game's graphics, however, aren't as good as CoD 4 overall).
This game is a fantastic game and a great thing to play solo. The multiplayer is fun as well, though not as refined as Halo 3's, which has the best FPS multiplayer on XBOX 360. It's good fun, just nothing out of the ordinary.
While its linearity and (sometimes) repetition dock its points a little bit, don't hesitate to buy this masterpiece. It's a great game, and I don't think there's a person alive (that plays videogames) that wouldn't like this. This is an experience no one can miss, if only to experience the sheer intensity of the single player.