The location is amazing. I stayed here for a month, and had a wonderful time. The Eiffel Tower is less than 10 minutes away (walking), there's the Invalides, the Rue de Cler, several delicious bakeries all around the building, a very nice drugstore right to its side (trust me, it's always good to have one of those nearby, specially during a pandemic), a couple of metro station within a less than 5 minute walking distance. The only downside of the region is that, because it is very central and touristy, prices can be more elevated than elsewhere in Paris.
The apartment, as most apartments in Paris, is small, but very comfortable. The sofa-bed really has a real mattress and you shouldn't have a problem with it - I am quite heavy and didn't feel uncomfortable there. For a single person and even a couple, it's quite comfortable. If I come back to Paris as a tourist, I wouldn't hesitate to stay there again. And the price is really fair for that region.
Their fan was far from ideal, so I went ahead and bought a new one. Hopefully they will keep it in the apartment, and that should be useful for future guests. Otherwise, it would become really hot in there.
Do not worry about bringing towels, pillows or any kind of sheets for your beds - they really have it all. There is also a washing machine inside the apartment, which is very useful and cost-effective, if you are staying for a longer time as I did. I didn't have any problems with the neighbors, which is always a huge win.
In sum: I do recommend this studio, either for a short stay or for a longer stay (if you are a single person or even a couple).