Playing with others in Demon's Soul (I like it if it's only 2 people instead of 3) is really great =D Sometimes I die on purpose just to play MP.
Black Phantoms are great.
Bad Company 1's SP was absolute crap, even with their attempt to put in a decent story.
I'm not sure about Bad Company 2's, but when I get the game, the first thing I'm gonna pop in is the MP, and then maybe play the SP a bit in between as breaks.
TBH though, since the first game's was such crap, I'm actually anxious to see how much they improved for the second one XD
It'll be a huge thing for Total War if they have a campaign MP (I heard something about it for Empire. Too bad my PC sucks).