I just finished Modern Warfare 2. It was, in a word, EPIC. The single player campaign is pretty short. I finished it on normal difficulty in about six hours. That being said it was a wild ride. I wasn't sure the sequel could live up to the original but it did and surpassed it. This game goes to great lengths to show you a realistic depiction of war while still maintaining that video game feel. More than once I felt uneasy about what I was doing in the game. I was also at times thinking "why the hell did I just go through that?" The plot is dense and twisting. The story was totally believable and there were parts that I predicted and many that I didn't.
I don't want to give anything away so I am being vague on purpose. You really need to experience it without prior knowledge to get the full effect.
My biggest complaint of the first one was how wave after wave of baddies would just keep coming at you until you passed some spot. That is fixed now, yay! I don't normally go for multiplayer but I have been playing the free for all mode and have been enjoying it a lot. I am going to try some of the other modes as well.
I got the XBOX 360 version so the lack of dedicated servers doesn't affect me. I can say though that so far online play has been smooth and easy.