First things first: the neighborhood is fantastic! Centrally located, restaurants/bars/etc, in Le Marais. One block away from the Metro. Excellent patisserie/boulangerie right across the street for breakfast. You cannot ask for more--if you want a convenient launching pad for your Paris explorations, this is the spot. We walked to many tourist attractions.
Communication with Thierry before and during our stay was quick and responsive, though we never met him in person. When we arrived, Thierry asked Josie to meet us. She showed us around the apartment (she speaks little English but we figured it out). When we left, we just left the keys inside and closed the door.
The building and apartment are charming and comfortable. The apartment is one flight of stairs up from an interior courtyard and feels safe and secure. The courtyard is really lovely, painted white/blue and with lots of plants. The apartment is small and furnishings are clean but basic.
A few minor issues with the apartment that we noticed: the windows have no screens. There is an air conditioner, but it must vent through an open window--which means, without a screen there, you must risk inviting the mosquitos in if you want either fresh air or air conditioning. Fortunately, it was not so hot during our stay, and we opted to keep the windows closed. The window shades could stand to be replaced with something a bit more light-blocking, too. At night, we wore eye masks to block the light coming through the window next to the bed.
Overall, the apartment worked well for our purposes--we wanted to get out and see Paris, so the location allowed us to do that easily.