Laëtitia and Cécile were very responsive in the time leading up to my trip and were happy to arrange a way for me to check-in early. Their place is cozy and very lovely. It is a small studio with a pull-out futon and might be cramped for more than 2 people, but my friend and I had no problem with that.
The subway station is just a few minutes away and whisks you right into the city within 10-15 minutes. There is also an outdoor market by the station that set up a few times during our stay, with vendors selling fresh produce, fish, cheese, and other artisanal goods. If you don't feel like taking the metro, within a 10-25 minute walk, you can be in the heart of Marsais or find yourself strolling along the romantic Seine River towards Notre Dame and the Louvre.
The only possible downside I can see is the flight of stairs up to the apartment. Like the description says, their place is located on the 6th floor and is definitely a long, healthy trek up and down the stairs. I particularly enjoyed the walk up and down, but it definitely took the wind out of me every time!
I highly recommend Laëtitia and Cécile's place for anyone's stay in Paris.