Alex is a fantastic host and our stay on his property was great. I highly recommend.
Comfortable:
The apartment is comfortable, scenic, and very quiet compared to most Parisian apartments. You can't get closer to the heart of Montmartre since it is around the corner from Place du Tetre. The bed was comfy (a huge point for us, since I have a bad back), view is nice, and the apartment has all the basic amenities (good kitchen, bath, internet, and a washer!). Despite it being a warm summer, the apartment stayed cool throughout the day.
Location and accessibility:
Despite it being on top of the hill, we never had a problem going up and down. The apartment is a few blocks away from the Montmartre 'Funicular' (basically an elevator), and most Ubers could pick you up without a problem. The metro is also easily accessible so you can travel to other parts of the city relatively quickly.
Service:
Alex was super responsive from our first contact, handing over the keys, and giving them back. One special note: I did something stupid--locked myself out of the apartment despite him giving us two sets of keys. Alex was quick to respond to our call for help (even though he was having his own holiday), and helped arrange for a locksmith to come by at an affordable price (they are known to scam tourists). So Alex--lifesaver.
I strongly recommend Alex and the apartment in Montmartre and would stay again next time we visit the city.