First and foremost, the location was great. As other reviewers have stated, steps from one line of the metro and 7 minutes walk from two other lines including the line to Gare du Nord and on to Charles de Gaulle from there; very convenient! There's a market not even a block away, a pharmacy just out the door, plenty of restaurants and coffee shops, and you're right on the edge of the interesting Marais district. One reviewer commented that they felt far from top sites, but we went all over the City (walking and Metro) and found it all very accessible (Notre Dame, Eiffel Tower, Arc de Triomphe, Sacre Cour, etc.). We walked up to the canal, down to the Seine, all over the Marais and over to Pompidou, etc. (And not to worry about the bar across the street - just close the shutters if it gets loud.)
Secondly, the rooftop view, the tiny wrap-around balcony, and the overall ambience and charm of the place made for a very Parisian feel. Such a delight! Very spacious too, though the way it is laid out, I'd say the apartment maxes out at 4 people.
Thirdly, Pascaline was a warm and accommodating concierge who spoke flawless English and remained available by text during our stay. She even went out and bought a couple more pillows at my request (bringing the total pillows to 6 for the two bedrooms) and she took lots of time to explain the laundry to us and offer neighborhood tips.
Fourth, the apartment had all the basics you might need.
Fifth, because of a smoke alarm (someone had burned their morning toast!), we met some of the charming neighbors and they all seemed very happy to have us. We felt at home.
That said, I agree with other reviewers that an investment of under $1,000 would make a real difference. When the owner relocated , my guess is that she took her comfortable chairs and sofa with her. Go to Ikea and buy a small sofa and an arm chair; there truly is no comfortable place to sit, which made challenging when we wanted to stay up chatting together at night. Likewise, the issue with the handheld shower easily could be addressed with a hook. (Tip: I made my own hook, by opening the window and propping the shower up in the window hardware - -this gave me a proper shower, and I was even able to sit in the corner with warm water streaming over me. Downside is you have to have the window open!) Lastly, add a few rubbish bins - would be useful to have one in the bathroom and each of the bedrooms.
Final point: when we were there, the radiator in one of the bedrooms was not working. Pascaline pointed out that there was a radiator in the hallway nearby, but if you want privacy and quiet, obviously, you need to close the bedroom door. On the bright side, each bedroom has a very warm duvet. Just make sure that heater has been fixed before you arrive if you're coming during the chilly months like we were (end of Feb/beginning of March).
Overall, we were very pleased with our stay!