This location CANNOT be beat! It's an easy walk to great neighborhoods on both the Left and Right Banks (5th & 6th, Marais and Bastille) and the Ile de la Cite (Notre Dame's island). It's also a short walk to several Metro & bus stops for getting to spots farther away, like the Eiffel Tower or Champs Elysee. The Ile St Louis itself is lovely with plenty of shops and restaurants as well as a small market for basics like yogurt, milk, and fruit. There are also a couple of bar-boats that remain docked just below one of the bridges, which is a fun way to relax along the water while having a snack & drink.
We visited in August & temps were in the 80s, but the apartment was very comfortable due to large windows & fans in every room. Plenty of space for 2 adults, 1 teen & 1 toddler. Washing machine in apartment; no dryer but extensive racks for hanging dry (just be sure not to leave your wash until the night before you leave).
I would note for awareness that the elevator was broken during our stay. The apartment is 5 floors up (in France they don't count the 1st floor, so it is the 4th floor, but you go up 5 flights of stairs), and each floor is separated by 20 stairs, so if you are dependent on the elevator, I would suggest inquiring before booking to confirm functionality. Also, it is TINY! Could maybe fit 2 people and a suitcase.
Mathieu was easy to correspond with and very responsive leading up to & during our stay. Although he was out of town, there was a caretaker for the building who let us in & who was available throughout our stay should we have needed anything. All in all, a fantastic stay in a truly unique location. Highly recommend!