I will start by staying that the apartment itself is quite sweet and in a fairly central location. It's also busy until pretty late at night, which made me feel safer as a solo female traveler. When you stay here you really are paying for convenience.
However, I did run into a few problems on my trip. The first time I took a shower, the top floor of the apartment completely flooded because there was hair in the drain from the previous tenant that wasn't cleaned out and the water built up. This was a cleanliness/amenities issue that I thought was within airbnb policy to cancel over but my request was declined. The shower also consistently ran out of hot water about ten minutes into each of my showers. I did bring this up with the host, but it wasn't really addressed. I know buildings in Paris are quite old and I was prepared for that; I wasn't expecting a five star hotel experience, but these were definitely significant inconveniences. Additionally, the host was pretty unresponsive. It would take 24-48 hours for him to respond and eventually he stopped responding altogether. To be honest, at times he came off as passive aggressive. I am willing to give the benefit of the doubt and assume they were having an off couple of weeks, but it didn't make for a pleasant experience.
Again, the apartment itself is fine. As others have said, it's quite small- the pictures make it look slightly bigger than it actually is. I had to leave my luggage downstairs because it wouldn't fit upstairs. Also again as others have said, if you have a lot of luggage I would choose somewhere else as there is no lift and the stairs are pretty uneven. These are all made pretty clear in the apartment description so it wasn't a surprise, just my experience.