The reason it became a phenomenon
If muitiplayer it's your cup of tea, as for now, 2012, the servers are still running with quite a community presence. The formula works pretty well, and they way the level up system works make it very hard to get away from online play.
I do appreciate single campaign, and the Call of Duty franchise always manages to put out a strong title, that's critically acclaimed and for it's high sale volumes and the genera that it became and set the tone for the contemporary video game industry. Modern Warfare is a must play FPS title.