Same thing, I did not format it as a letter, but rather as a small monologue with my future roommate. Don't forget, though, that this essay is meant to be focused on YOU. So speak about you, yet beware of sounding selfish. It is almost the only place where you can share some 'I's for 'You's, so don't hesitate doing it! (That's, IMVHO, the directives I used).