The apartment is on the 6th floor of an old building. There is a tiny elevator retrofitted in the middle of the staircase. Two, possibly 3 people can squeeze into the elevator. Overall, it worked well, but if you're traveling with kids and have a large stroller, you might not be able to fit unfolded.
The apartment is small. We mostly ended up hanging out in the kitchen, where there is a small foldout table and 2 stools. The kitchen is very well equipped, with coffee, tea, electric stove, dishwasher, and a washer/dryer unit.
The bed was very comfortable.
Note, there are no window guards or mosquito nets on the windows, something to note if you're travelling with kids.
In terms of decor, the place is a hodgepodge of old furniture and IKEA. If you're looking for an apartment that feels Parisian, or cool, it's not it. But everything is functional.
Communication with Fernando was excellent. Everything went pretty smoothly. Gaia even provided a crib for our baby for a small fee.
Location is very good. Not touristy, yet you're close to Le Marais, Notre Dam, and the Louvre.