I don't want to start by comparing this with MW2. As a game in itself it is good. The weapons and ideas are modern and feel professional. The multiplayer is smooth and you will enjoy this game a lot. Comparing it to MW2, I would say the only reason to buy it is for the tonne of new multiplayer maps available as you can get very bored running round the same environments. Its great to play this new installment because people dont know the maps well enough to camp at hotspots. Most people in MW2 can throw a grenade from the other side of the map and kill someone because they have practised thousands of times. This novelty will wear down. There is a great range of playing styles - deathmath, kill confirmed, infection etc.
Negatives - The gameplay is really smooth, but there are still a few issues with running (having to slick down is a pain) and the servers lag quite a lot which gives a distinct advatage to the other team. There is a massive innacuracy when using weapons. It may be down to lag (a commone issue you might have guessed) but the shooting is not very balanced and often when attacking one vs. one, its pretty much luck on who wins. The stab kills instantly even if a shotgun is fired. There is amassive error in respawning too, you can respawn right next to an enemy in multiplayer which is rubbish for a game where one hit kills.
Overall - If you have a choice buy MW2 for the main story line, its excellent and wait till this is really cheap and class it as a 'patch' on MW2 that does nothing differently, just add the content they probably wanted to in the first game (but ran out of time or budget). If you already played MW2 then play Battlefield till this is cheap. If you are a fan you will have this already so you wont be readiing!