My boyfriend, his parents and I stayed at Julien's apartment during one of the hottest weekends of the year! Luckily, because of it's location in the quiet courtyard, it being on the bottom two levels, and in the shade, it stayed nice and cool. Julien provided a fan for the upstairs, which made it comfortable also to sleep. He's a very communicative and nice host! The apartment is not only decorated beautifully, but also in a really fantastic location.
Having lived in Paris before, I wanted somewhere that would be great given our short time span, and this apartment was that! A 3 minute walk to Hôtel de Ville, 8 to Châtelet, and in the center of the Marais, we really couldn't have asked for a more well-connected and central place. Even though it's in a busy area, the apartment is tucked away behind buildings off the main street, with a view of the Centre Pompidou from the upstairs window, and so it's also quite quiet. It also is neat to walk down the end of the path, through a bunch of old, connecting buildings, which made us feel the history of Paris!
The only negatives would be that the bottom level was a bit dark from the front, there was a skylight above the fireplace but it is located between buildings, but again as it was so hot outside for us that wasn't a problem. Also you have to pay attention when opening the window in the bathroom, because it slams easily into the (really nice) rain shower head.
We would certainly stay here again, and couldn't recommend it more!