Well after just a few hours of playing it, I finished the campaign in Call of Duty 4: Modern Warfare. I finished it, ugh, the name of the difficulty escapes me; let's just call it the second dificulty. Which wasn't perticularly difficult 
. After doing so, I looked up what acheivments I had missed. Appearantly, the next step is to beat it on expert level. I'll get to that soon enough.
Onto the multiplayer. It is by far some of the best multiplayer I have ever experienced. I keep relating this game to Battlefield 2 (PC version), which is still my number one online multiplayer game. The experience is close to BF2's. There are various game types to choose from, most that you must unlock. You unlock game modes and other nifty prizes through gaining military ranks. You can unlock new guns, scopes, lasers, and other things to help you in the battlefield. The only downside I found was the medics and vehicles/jets/choppers; there aren't any. I had such a great time playing as a medic in BF2 and flying choppers with my friends gunning people down was such a great part of Battlefield 2. I suppose you could excuse the lack of a medic class with the fact that the respawn is fairly quick. So, as I gain more ranks and unlock new cool things I'll be sure to play this game for hours on end.
To end this short review of mine, let me say that this is my favorite and, in my opinon, the best Xbox 360 game out there. Many people would compare it to other shooters on the 360 and when I compare it to Halo 3, it comes out on top with a foot firmly planted on Master Chiefs face.