In one word--LOCATION!
Located in one of the most interesting and attractive quarters of Paris! Minutes from Norte-Dame and the university, with almost everything you could wish for (grocery / pubs / coffee shops / pharmacies) just around the corner!
Stephane was great and quite flexible meeting us around 1am when we arrived from our flight to Paris and gave us some useful tips for our visit.
The apartment is very nice and well facilitated--great for spending quite a few days in Paris.
Bed setup is interesting :) you need to hop over or use a step to climb up.
Definitely recommended!