I don't really like watching movies with friends because it kills the sense of immersion and suspense of disbelief. If I'm trying to watch a serious movie, and a bunch of guys go "OHHHH!!!" at a gory scene and high five each other, I can't really feel the horror of the situation.
Single Player experiences can be golden. Killzone 2 singe player is awesome, with all the explosions and effects going on.
Games like Total War I feel aren't going to really work out well in multiplayer (that is, in the outside campaign mode. Not the individual battles), even though it'll be awesome if it can.
Co-op isn't necessarily any more fun than single player IMHO. It depends on each person's preference.









