It is the best FPS multiplayer experience I have had. Yep I said it.
I find almost every multiplayer game insanely boring and only normally play singleplayer, MW2 was an exception.
Why? Because there is a sense of progression, something I need in games. A feeling that A) I am either growing, advancing and becoming better/stronger, or B) following a storyline of some kind and looking toward a particular goal. One or both of these things are always in the Singleplayer of shooters, and hardly ever in the multiplayer. Modern Warfare 2 is an exception to that rule and actually incorporates point A in it's multiplayer. When I played it I advanced, gained ranks, got better weapons and upgrades and there is actually a POINT to playing the game. Making it fun and addictive at least in short bursts.
It just proved I was only playing the game for the sense of progression once I unlocked the akimbo shotguns and there was nothing else I was interested in unlocking.. I immediately felt bored of the experience and didn't play it again. A few days later I uninstalled it from my PC. But I racked up 73 hours in the game altogether (completely unprecedented for me, since I am the type of person who plays games until the end of the storyline then uninstalls them, so the average amount of time I play one game is about 8 hours)
Its not a great game and it has major problems, but just such a simple addition as a leveling system kept me coming back for more and made the game incredibly addictive despite how much I could criticize all other aspects of it at every turn. For this reason I will keep buying Call of Duty games and I really hope other FPS games start taking a leaf from the COD book for their multiplayer (it seems as though this may be happening, since Bioshock 2 included a similar leveling system I believe)