First and foremost: this is NOT a place for two people: it's super tiny and the bathroom has no door. Thankfully I was by myself :). And, if you are a tall person, you'll probably want to stay somewhere else. The slanted roof was hard to navigate even for me at 168 cm/5' 6".
Another downside: no WiFi or coffee machine. They are explicit about that, but of course I did not check on WiFi beforehand because it seems like a given.
On the bright side, the apartment is immaculate: reallly clean and otherwise well equipped, charming in how well the space is solved.
The agency that manages the apartment is super flexible, but of course you don't have the availability of a real host - sometimes it took them a whole day to respond, which wasn't very convenient when I was trying to check in and forgot the codes that I needed to get into the building.
I didn't love the location because it's in the most touristy square of them all - to see real Paris you need to walk away from that, but that's OK. It was NOT loud though, you could sleep well.