Cristina and Matteo's apartment is small and perfect. It is exceptionally clean and the design optimizes the available space. The bed is comfortable and, during the summer, there is almost always a lovely breeze that keeps the room nice and cool at night. When you wake up in the morning, you can go down to the street, walk towards the Seine and thirty seconds later you're facing Île-Saint-Louis with Notre Dame cathedral to your left. The only drawback some people might find is that it is located on the sixth floor, but one gets used to the climb very quickly and (really) if you're in Paris for a short stay you'll only be taking these stairs twice a day (once in the morning and once when you're too tired to continue seeing the city).
As hosts, Cristina and Matteo are friendly, generous with advice and happy to help ensure your stay is comfortable. I lived a month in their apartment and it was just wonderful.
I would recommend these hosts and their apartment to anyone without hesitation.