Very pretty apartment among the rooftops of Paris, it was an apartment I would dream about living in when I was younger. It is very much a lived-in apartment by the owner when in town, so don't expect it to be just a property used for renting out; the owners possessions are very much in the apartment during one's stay (unlike some places that are used strictly as rental properties).
Another reviewer commented on the heat; I can imagine in the summer it would be very hot as the temperature inside was not well insulated. During my stay it was perfect (50s at night, 70s day), and the windows let in nice air. There is no AC (typical of Paris). The elevator doesn't work so be ready for 6 flights, which I didn't mind. The views were lovely.
While not in the "center" of Paris, it is walk-able to most nice areas. I felt safe and there were many conveniences nearby. I was attending a conference at the Institut Pasteur, so the location was ideal.
The bed was VERY comfortable, and the room was quiet (with the windows closed). I slept great.
The hairdryer broke while I was there and Karim graciously brought me a back up.
Pros: safe, pretty, comfortable, spacious "bachelor" pad with wifi, fully functional kitchen, non-smoking. Water pressure was excellent in the shower (something important to me, I guess!).
Cons (which were minor): walk up, could have been a little cleaner (floors and shower), cable TV not working at the time, needed more toiletries (facial tissues, paper towels in the kitchen, an extra bath towel--I stayed 1 week and had 2 bath towels, 3 hand towels, no washcloths), no AC or window screens (but this probably is only an issue on the hottest days).
I would definitely stay there again.