The gist: The location of the apartment was perfect, the apartment itself lovely, but kind of dirty and very hot.
The neighborhood Marais is one of the loveliest places to stay in Paris, so just the location alone makes up for a lot. The apartment itself is lovely and has a great layout, just be aware, that there is a gap in the wall to the bedroom. In the living room it got really hot during the day (even on days when the max temperature was 26 C) and there was no fan or cooling down until the evening, when the sun was gone.
The biggest downside was, that the apartment wasn’t clean. The fact, that we got to check in early didn’t have anything to do with it. There was what felt like dirt from a long period of not cleaning properly (stains on the bathroom door, dust and dirt on surfaces) plus there were stains on the towels.
All in all we still enjoyed our stay, but it could have been better.