Stayed here 4 nights in April with my husband, our 5yo daughter, and my husband's brother.
The location is good--close to several Metro lines and walking distance to the Louvre, Arts et Métiers, Gare du Nord, La Samaritaine, etc. The check-in/check-out process is very easy and communication with the host were great. The apartment is up one flight with no elevator but we have no mobility issues so it wasn't a problem for us. It was clean and modern (except for a very worn sofa) and the kitchen is well-equipped.
We had 3 problems--odor, sinks, and neighbor noise. One of the bedrooms is an interior room and smelled like something died in there when I opened the door in the morning when my daughter woke up after our second night. I alerted the host and they came to deodorize the room, but the issue was not really resolved. They left an odor-eliminating substance in a plastic container that smelled too strong, and although we left the windows and room door all day, it still smelled awful after a night of sleeping with the door closed (even with the air cleaner in there on high all night). The bedrooms with windows only had light-filtering curtains, which is why we opted to put our daughter in the interior room.
As mentioned in a few other reviews, the sinks in the 2 toilet rooms and the en suite bathroom are ridiculously small. Even smaller than airplane sinks. We found this pretty frustrating as you could barely fit your hands under the water and it was near impossible to rinse soap off your hands without touching the dirty water that had not yet drained.
Lastly, the upstairs neighbors are noisy at all hours. I recognize this is out of the control of the hosts, but I think travelers should be warned. It sounded like they were walking around with lead boots on, doing light construction, and moving furniture until 2:30am. The noise then resumed around 6am. I'm a light sleeper some nights, and I found it hard to fall asleep.