Mercedes' apartment is super well placed, close to supermarkets, bakeries, two subway stations (Nation and Reuilly-Diderot), restaurants, everything one might need during a stay in Paris. Just 2.5 km is the Seine, where it's possible to go for a run and enjoy the lovely view.
Regarding the apartment in itself, it is proper, it has a full kitchen, with all of the basics for the guests. Wifi works properly, the working station is a plus for those who have to study or work during their time in Paris.
One thing that is not ideal for longer stays is the sofa-bed. For a few weeks it is more than fine, but for longer stays it might become uncomfortable.
Other than that, communication during all times was easy. Mercedes was always polite and helpful. During the check-in, I had a small issue, but it was solved as soon as I informed her, who responded promptly.
As a last information, it is on the second floor (it has a small terrace, good for summer times) with no elevator (normal for Paris), which for me was not a problem, but for those with reduced mobility it might be an issue.