I haven't really played multiplayer on Halo, because I usually just stick to campaign. Co-op mode's cool, because when I play with...
a)... my brother, we just blast through the levels and kill everything (he rules at FPS games)
b)...my cousin, I have so much fun because we just stuff around (she tried getting a tank through a narrow crevasse by lifting it off the ground with a grenade!!!)
And I don't get time to see cut-scenes (as much as I want to, my partner is too eager to keep playing), so I can't really judge too much on the storyline.