We were a family of five (two adults and three teens) who spent about 10 days in the apartment and we had a great time. The place is spacious, relatively quiet, well-heated, and fully supplied. There are two full bathrooms (spread out in four separate rooms). All communication with Francoise was quick and delightful.
There are two things to be aware of. First the kitchen is small, although it was fully adequate for our needs. Second, it is located on a staircase. This is totally charming but also resulted in a lot of stair-climbing during our trip (the apartment is on the sixth floor and there is a very small elevator you can use when inside the building). Half a flight up leads to Place Terte; a flight and a half down to the groceries, bakeries, etc. You can walk to three Metro stops, two on the 12 line and one on the 4. At the closest stop, Lamark, the elevator is out for renovation so that adds on quite a number of additional stairs. We like to walk and didn't mind it but it may be difficult to some, particular with strollers and the like.
Montmarte is a lovely neighborhood and the apartment is in the shadow of the Sacre Coeur. It was a great base for us to explore central Paris as well.