Visiting with my girlfriend in late September - we found everything to be perfect including the apartment and neighborhood.
Martine was extremely helpful waiting for us when we arrived to show us around the apartment and give advice on local spots.
The apartment is situated very close to the Sacre Coeur but in an area which is not mobbed by the site seers. Some of the best meals/wine we had in the city were nearby in Montmartre including: Le Jardin d'en Face, Le Petit Moulin & Le Grand 8.
The apartment itself is extremely clean and fresh. There is a ton of lighting and its a pleasure to wake up to every morning.
Finally, the balcony gives a view of the entire city and we sat out there every night. I proposed to my girlfriend there on the first morning - so clearly we loved it!