We hit the apartment at a bad time, right after the prior tenants left a couple of things broken. These were minor inconveniences.
You really couldn't find a much better location. I really liked being near the Rue de Mouffetard and the Polidor. There was generally some noisy conversation from a bar across the street until 1:00 or so; if I closed the front windows it wasn't a problem, but reduced the air flow. The back windows overlook a small courtyard. I would still say it's a fairly quiet apartment by Paris standards.
Honestly, the worst thing about the apartment--which I had fair warning of--was the five flights of steps. (They're plenty spacious; just a long way up.) I work out and everything, but I never looked forward to heading up those stairs. I would recommend that anyone with a heart condition not rent the apartment, and if you've got a lot of heavy bags, you're going to need some rest breaks on the way up.
On the other hand, you can see Sacre-Coeur off in the distance. . . .