The apartment is very small, but very well equipped. It is sunny and silent, because it exposes towards the inner courtyard. The building is in good conditions, but the stairs are ancient, so quite narrow (a little uncomfortable if you have huge suitcases).
It is really in the center, next to Pantheon, and ten minutes walking from Notre Dame.
In general, I would suggest Marc's apartment, but only for single person, because I think it would be a little uncomfortable for a couple. For example, there is no sink in the bathroom, which is very very small, so you have to use the one of the kitchen. Moreover, the table is a small desk, not really suitable to eat on, after all if you are in two. Anyway, Marc is very kind and ready to help you for everything you need, the apartment is clean and the position of the house is perfect to sightseen Paris!