My first roommate was the polar opposite of me, as you could tell by the statue of the Virgin Mary on her side of the room and the shirtless Jim Morrison poster on my side.
She wasn't a bad person, we were just utterly mismatched. I think the best thing is to find a roomie who you're congenial with, but not BEST friends with. All the best-friend roomies I saw ended up at each others' throats.