My father, sister, and I stayed in the apartment in November 2009 and found it to be delightful.The pictures are exactly what the apartment looks like, but for people who aren't used to Parisian (or New York) apartments, you might imagine it is larger than it actually is. It was a little tight for three adults (but we're on the large side - the smallest of us is 5' 10") but not uncomfortable in the slightest. I could see it being perfect for a couple, a couple with a child (or two smaller ones) or 2-3 friends.
The stairs - 4 flights, are only an issue if you are mobility impaired (there is no elevator). My father, 65, pleasantly plump and sporting an artifical hip, never complained. Now, my sister and I did whine a bit when we carried our luggage up the first day, but the rest of the time it just helped me burn off the cheese and wine calories.
The owners really have thought of everything: travel books, umbrellas, shower cap, ironing board, even spices. Every time I thought "oh darn, I didn't remember to bring X," I looked around and found it already provided for me.
The beds are comfortable, the water is hot and has good pressure, the building is quiet. It's really perfect - especially if you don't plan to eat every meal in a restaurant.
The neighborhood is charming and very safe, with a 2-minute walk to Rue Cler (fantastic cheese and wine shops, yummy crepes and on market day - fantastic people watching and browsing) and a few minutes more to the Metro and restaurants that stay open pretty late. There is a great little health-food store near the Metro stop, which had gluten-free food (important for many these days) and a great Italian deli between the Metro stop and the Seine for a quick picnic dinner (if you ever decide to eat something other than French food. :)
The owners refunded my security deposit very promptly and the entire thing went without a hitch. The owner's representative was very friendly and accomodating as far as when to meet us when we arrived. Overall, I highly recommend it over a hotel and plan to stay there again soon.