We stayed at Nicolas' apartment for 6 nights, and it was a great base for our first visit to Paris! This apartment is quite tiny, suitable for two people maximum. The bed is a pull out couch, which is surprisingly comfortable. Once the bed is pulled out there is practically no walking room in the main room, but you probably won't be spending too much time in the apartment anyways, and I think this is a typical size in Paris. We were able to cook a few meals in the kitchen, as you have access to pots, pans, etc. The wifi had a fast connection, and the tv had a few English channels. I also loved the way the apartment was decorated - very simple and classic.
Nicolas was a very good host - always answered any questions that I had within minutes. He even offered to show us around the area, but we were too tired from our long flight to take him up on the offer.
The location is about a 5 minute walk to the Pigalle metro station, which can bring you all over the city. The only negative is that you have to walk through the red light district to get from the metro stop to the apartment, which can be a bit uncomfortable at night.
All in all, if you don't mind staying in a small space, then this is a very nice apartment with a great host!