Marie-Pierres's apartment lays in the heart of Pigalle, just around the corner from Montmartre. All in all I stayed there for about 4 weeks while attending a summer course.
Pro: The location is amazing - lots of great restaurants and bars around and two metro lines that basically guarantee being able to get around Paris in max. 30 min. There are also a bunch of supermarkets close by and a cute little street where you can get whatever you need really. Marie-Pierre made sure to get me the keys, although she herself was not present in Paris at the time, in that regard she really went out of her way. She also allowed me to stay a couple of hours beyond my official stay to ensure I didn't have to carry my luggage around the city until my train departed.
Con: Although the place seemed very clean at first glance, after being there for a while I realized many blind spots being not very clean (under the couch, under the bed, in the box with extra bed sheets, etc.). When my girlfriend came to visit (she has a dust allergy) there were some major problems with her allergy. Additionally, upon arrival there was exactly one trash bag in the whole apartment and one roll of toilet paper. While the kitchen has everything one needs for every day meal prep, don't try to get fancy - there is no oven and various kitchen utensils are hard to come by.
Other: The communication varied from great to rocky - sometimes answers came more or less instantly, other times it took longer than it (in my opinion) should've. It is also important to note, the apartment is on the gorund floor. Meaning at night it can get quite loud right outside the apartment (young people going out, etc.). For me this was no problem, as I am an easy sleeper but might bother some people. However, feeling safe was never an issue in that regard.
All in all, I would say that the apartment fulfilled its duty and gave me a place to stay that I enjoyed coming back to. Cleansliness could definitely be improved, but the great location definitely makes up for it (to a certain degree).