I absolutely loved my stay at Caroline's place in Montmartre. I never met Caroline, but she was responsive and got back to me quickly.
I lived in Caroline's apartment for an entire month, and would absolutely recommend it for long-term stays, solo travelers, couples, etc., as well as shorter-term stays. I am a solo female traveler and never once felt remotely unsafe or uncomfortable.
The space is light-filled, quirky and artsy in design, has a combo washer-dryer in unit, and has a kitchen that allows for cooking at home. The balcony is lovely and the internet worked well, once we sorted out a small snag about the network I was supposed to connect to. It looks smaller in the pictures than it truly is (the bedroom is completely separate from the main room which has a small couch and two desks, and this room is again separate from the kitchen/bathroom/hallway area), and would be quite comfortable for two (though the bathroom is admittedly interesting, coming from an American who isn't used to a wet bathroom). If you are a light sleeper, you might be woken early by the traffic outside; being by a metro station, the mairie, and several busy streets, it's very bustling and busy until a bit late and starts fairly early. I absolutely loved this, as it feels very energetic and lively and there's a lot to see, but if you're a very light sleeper, perhaps bring earplugs.
The apartment couldn't possibly be more convenient, location-wise; there are tons of bars, restaurants, and grocery stores nearby, Jules Joffrin station (metro 14) is right across the street, and Simplon station (metro 4) is a 5-minute walk. It's about 15 minutes walking to get to the Sacre Coeur, and just incredibly convenient and a wonderful location overall. If you're not super familiar with Paris, it may seem far from some touristy things, but it's about a 20 minute (30 max) metro ride to all sightseeing spots, and walking through Paris is lovely too, so you can easily get everywhere on foot as well (...sometimes it'd be a long walk, but it's still a nice one!). Plus, Montmartre is, in my opinion, one of Paris' best neighborhoods—you could stay in direct proximity to the apartment and still have plenty to see, eat, and do.
Overall I had a truly lovely stay at Caroline's apartment and would stay again in a heartbeat. If you're just visiting Paris for a short time, I'm sure it would make a great home base, but I'd also strongly recommend it for someone who wants to "live" in Paris for a longer stretch and have somewhere that feels like home while they get to know the city. Thanks again, Caroline!