1) I bet you’d be in much better shape and go to the gym more because you’d have like a work out buddy trapped inside you. Because the demon would be like "There's no way I can rule the world in this piece of shit body!!" and so he'd be bugging you all the time to go lift weights and drink calf's blood.
2) You could be a lot more productive because one of you could sleep while the other one takes over your body and does the dishes and shit like that. Now when it’s the demon’s turn to vacuum or mow the yard, he might not do quite as good a job as you would. Demons are not known for their neatness (hey, who is?). But as long as you’re not the one having to do the chores, do you really care if the demon is a little sloppy or spews some vomit on the kitchen floor occasionally?
3) You could do all kinds of bad shit and blame the demon. Your girlfriend could walk in on you screwing a prostitute and you could just shake your head, open your eyes real wide and then say “Where the fuck am I?!?” Then you you act like you’re real surprised that the only thing you’re wearing is your socks and you’re doggy style with an 18 year old. "Sorry honey, must have been the demon". See? Problem solved.
I did think of one downside to demonic possession... If you wake up and your girlfriend says “You were an ANIMAL last night.” And you’re like “What?” And she goes, “We don’t usually do it in the middle of the night like that, but you wouldn’t take no for an answer! The way you were pulling my hair and kept calling me a slut was a total turn on! But you kept saying ‘Be quiet or you’ll wake him up’. What was that all about?” You’d be like, “Uncool demon, very uncool”