Cannot say enough good things about this apartment and the host. The apartment is massive by Paris standards -- 4.5 rooms: bedroom, common room, separate kitchen, bathroom, and separate little WC. Most everything was in very good condition and very clean. Comfortable large bed, plenty of storage, full size tub/shower, spacious full kitchen, huge windows let in a lot of light. It is on the third floor but has an elevator, all very easy. There is also a picturesque little private cobblestone courtyard, though there is a little construction there at the moment. We were there an entire week, and the apartment exceeded all our expectations.
The apartment is located on the north half of the Marais. It is on a little side street close to multiple main thoroughfares, safe but also secluded. The entire Marais is an easy walk away. Also a 15-20 min walk to Notre Dame, maybe 25-30 min to the Louvre -- Paris is very walkable and Le Marais is close to everything except maybe the Eiffel Tower, the Arc de Triomphe, and Montmartre (all of which aren't really close to anything anyway). Regardless the apartment is a 5 min walk to multiple Metro stations so you can go anywhere easily.
Also within a few blocks of the apartment: a great covered food market with fresh produce and also prepared food (Marche des Enfants Rouge), excellent restaurants, multiple supermarkets, pharmacy/drug stores, multiple good boulangeries, an excellent fromagerie, a cute neighborhood park, etc etc -- you couldn't ask for a better location.
The host was great also. Very prompt and good communication both before and during our stay. He speaks excellent English. Currently he is not living in France, but has some helpers who are available there and very accommodating, also good English speakers. Check in was easy and flexible -- our flight was delayed but it was no problem at all, and one of his helpers was waiting for us at the apartment to show us everything. Check out was also very easy.
So any negatives at all? Not really. The street outside is usually quiet, but you can occasionally hear some of the nearby nightlife, nothing too bad and pretty much standard for Le Marais since it has fantastic nightlife. But I am stretching -- it really is a gem. It will be top of my recommendation list for anyone going to Paris, including hopefully ourselves not too far in the future!