The strong point of the apartment is that it's very well situated near Jardin de Plantes (interesting museum, small zoo and nice botanical garden) and Rue Mouffetard (local restaurants and bars). The metro (Censier-Daubenton, line 7) is only at 5-minutes walking distance and this line takes you directly to Marais, Louvre museum, Hotel de Ville, Palais Garnier, ... The apartment at a first glance looks cosy with many books, old furniture and wooden beams, because the building is from the XVIII century.
However, the beds have very hard mattresses and poor pillows. It would cost little money to buy new mattresses and comfy pillows to give guests a much better sleep. The frying pans are worn out, kitchen equipment is old and dirty and left food from other guests mostly expired many years ago.
Many corners in the apartment haven't been cleaned for ages and we had the feeling that hosts don't care too much if guests are comfortable or not.
P.S. Normally I don't like to write negative comments, but I think that it's fair to warn other travelers about weak points.