MW2's MP is pretty good. Most of the glitches have been fixed now, so its not as terrible as it was when it first came out. But you will always have the campers, noob tubers, and the annoying ones who like to run around stabbing people. Needless to say, the game likes to piss you off. But if youre playing with a group of friends and dont mind listening to a bunch of annoying little kids or wannabe gangsters (since you cant do part chat on the majority of the playlist) then the game can be pretty fun.
BFBC2, while obviously still a first person shooter, plays completely different in my opinion. Ive only been playing it for the last 2 days since its been out, but i already find myself enjoying it more than MW2. There are vehicles for you to use which vary from tanks, jeeps, helicopters, atvs, boats, and jet skis etc etc which makes for some good times. The game could use more gametypes, but the ones it currently has are pretty fun. A few of my gripes are that it takes FOREVER to try to kill someone. But if you play hardcore it'll solve that problem. I.E: One hit kills with a sniper, even if its a body shot.
Since youve already played MW2, i suggest you try out the demo for BFBC2 if you havent already.