Location, location, location. I would not hesitate to stay here again because it is convenient to almost everything you will want to do in Paris, and it is also close to major transportation hubs if you need to go further afield. A big plus -- Nathalie is a wonderful hostess, with excellent communication. She explained everything about the apartment well, and I feel certain that if I had any issues, she would have helped (I did not have any problems).
This was a perfect stay for me, for one week alone in Paris. However, some things to note to decide whether it is a good match for you: There is no wifi, and you do have to climb six flights of a narrow spiral staircase, so make sure that is okay for you (and your luggage!). Also, the plumbing in the bathroom is old, and sometimes there is a slight odor coming out from (I think) the shower drain. It did not bother me and I could open the windows to air out, but if you are someone who is very sensitive to smells, it might annoy you.
That said, all of the appointments and furnishings were fresh and new, and the apartment was very clean. The balcony was an absolute delight. The space is quite small (I have lived in tiny apartments in both Japan and Hawaii, and this was even smaller!), so it is definitely for one person only; I would not attempt two. Despite this tiny size, there is everything you need for a brief, or even extended, stay -- electric kettle, microwave, fridge, hot plate, basic cookware and dishes... The only things I really used were the fridge and the kettle.
One thing to note about the bathroom, there are no outlets, nor any hair dryer, but outlets in the main room should meet your needs. Also, the hot water heater does not have much capacity, so expect to take brief showers where you may need to shut off the water while lathering, much like an RV or camper shower.
I had a wonderful visit and can definitely recommend Nathalie's place!