I lived in Xavier's flat for 71 days and I absolutely loved my experience here.
The flat is in the most central area of Paris, in the epicenter of the LGBT district, in an extremely lively area with pedestrians crowding into the streets at most hours. The moment you step outside the door, you are literally in the heart of lively Paris, absolutely bustling with action.
However, the flat itself is located in the rear of the building, facing an internal courtyard. Because of this, the flat is utterly silent at night. I want to re-emphasize this point for anyone who is sensitive to noise or a light sleeper (like myself). Despite the central location, this is an extremely quiet apartment, with literally ZERO noise from the street. Truly zero. I slept with the windows completely open for several nights and never heard a sound. The central location, plus the quiet, makes this a rare find.
On one occasion, the keycode to the front door stopped working. I sent Xavier a message on AirBnB and he arrived less than 20 minutes later to solve the problem. Amazing. Such great customer service to solve this problem so quickly.
The flat itself is obviously a studio - it is decorated modestly but tastefully, with lots of art on the walls. I lived here alone and found it perfectly adequate and charming.
As others have mentioned here, the toilet area is small, which I would describe as being about the size of an airplane toilet. However, the shower area and the sink are located in a separate room with a separate door, and the shower is plenty big. I am 6'6" (198cm) and I couldn't touch the ceiling in the shower, even if I extended my arms completely upward. The bathroom situation honestly never bothered me, but it's worth mentioning in case you have fussier taste than I do, so that you can make the most informed decision for yourself.
Overall - five stars. I am so thankful that I rented this unit, and that I lived in a place that maximized my ability to enjoy and explore Paris.
Thank you, Xavier.