I stayed four nights at David's apartment. It is in a typical old Paris building on the sixth floor with a winding staircase and a tiny elevator. The flat is carefully and beautifully designed for efficiency: the built-in kitchen and the small bathroom have all amenities.
There are two balconies that let in the gorgeous Paris light with doors that open to the air. One can see the roofs of Paris and the Eiffel Tower in the distance. Chief among the benefits of the apartment is the location: four blocks from the Metro; with a bus stop at the front door; near tons of restaurants and bistros; walking distance to Notre Dame, the Luxembourg Gardens, the Pantheon.
The only drawback is if you have mobility issues: the elevator does not service the final flight of stairs to the apartment nor the first several steps up to the elevator on the ground floor, and the stairs are long and curvy.