The only roommate I've had was my ex. We moved in together and I pretty much broke up with him in about 2 months, which sucked because then we had to spend the next 6 months together. Living with him wasn't that bad because we worked opposing schedules, so I rarely saw him. The problem was he was like a 5 year old and never picked up his own messes or anything; I was pretty much left to take care of the whole place.
It didn't really bug me too much until it got to the point where I could basically come home and tell what he had been doing all day based on the shit he left out.
Towards the end of it, I got to the point where if he left something out (cups, chips, plates, etc) I would just put it on his bed. At first I just left notes on his bed saying things like "Mark, I started the dishes before work. Please empty it when you get home," but the next night I'd get home and there would be a crumpled up note on his bed (ie he slept on it) and a dishwasher full of dishes.