There is so much to recommend this apartment, but there were also some big problems.
The good things:
The apartment is very big (especially by Paris standards) and beautiful. It was very comfortable and a nice place to come home to after a day’s site-seeing.
In all the best ways, it is clear that people actually live in this apartment, and it is not merely a cash-cow for some big real estate corporation. The apartment is charming and French and welcoming.
The location is amazing, in the lovely Marais neighborhood that we love so much. There are great restaurants and bakeries right nearby, and lots of shopping and museums and bars, and the metro is not far. It is a wonderful area in which to stay.
The location is also right in the center of the city, making it easy to get anywhere we wanted to go either by Metro or by foot.
The one time we spoke with the owner, she seemed very friendly.

The problems:
The owner has hired a management company to handle the AirBnB rentals, and so the owner was almost entirely unavailable and unhelpful. And, worse, the management company was completely incompetent in every way.
Check-in was ridiculously complicated and ended up being a nightmare. No one was scheduled to come to the apartment to help with check-in. Instead, I was supposed to go to one arrondissement to pick up keys from a lockbox at some random laundromat, and then travel to a different arrondissement to let myself into the apartment. Even if this plan had worked, it would have been a big inconvenience to drag my three luggage around to two different locations. However, it was even worse than that. After arriving at the random laundromat, there were no keys waiting in the lockbox. And there was no way to contact the owner, because she had abdicated any responsibility for anything. When my brother finally figured out a way to contact the management company, they were completely incompetent. It had rained, and I was left standing in the wet streets with my luggage for almost two hours before someone showed up to give me a key. No one ever came to the apartment with me to explain anything.
Then the second big problem occurred three days before we checked out. The internet died, leaving us without any ability to use computers or the television. Since I had work to do on this trip, this was a problem. Again, there was still no way to contact the owner. And, again, the management company was incompetent. It took multiple emails and phonecalls, and 12 hours, merely to get them to contact me. And in the end, they did nothing about it. I asked if they could bring over a temporary device (a mobile hotspot, or Wi-Fi USB dongle, or something) but they said the owner refused to pay for that. What?? Instead, I was told to wait for the internet company to fix the problem. (Which never happened.) Finally. 24 hours after the problem began, the owner finally deigned to contact me, and while she appeared to be pleasant, she ended up being useless too. No aid was offered, and no discount was offered. (Although, several days after we left the apartment, she did make the useless gesture of promising a discount if we ever stay with her again.) And the internet company never fixed the problem, so for the last three days of the trip, there was no way to use a computer for work or watch tv at night.
This apartment is lovely, and the neighborhood is amazing. But the owner needs to take more responsibility for renting her apartment and/or change the management company she uses. An otherwise very lovely trip began and ended very badly.