Ferdinand's apartment is the perfect place to stay if you want an authentic Paris experience. The views of the city from the balconies are incredible (totally worth the walk up 6 flights of stairs), and the bed and couch are comfortable. The apartment itself is fairly bare-bones, but you are walking distance to a grocery store, tons of restaurants, and lots of trendy boutiques - so there's no reason to just be hanging out inside all day ;)
Montmartre is absolutely gorgeous, and you are also just steps from the Pigalle subway stop, which will quickly and efficiently take you anywhere else you want to go. I would recommend this apartment to anyone who can brave the climb... it's a beautiful spot!