This is a charming flat full of character. It's true that it is up 6 flights of winding stairs! however by the end of our week's stay we had got much fitter, so that could be regarded as a bonus :). As soon as we opened the door, we knew the climb was worth it. The stairs are interesting winding wooden ones, too, not soulless dull ones.
Also - the location is perfect. The nearby food market and all the many shops, bars, cafes and restaurants are worth spending a week exploring on their own. The flat is in easy walking distance of some great small museums as well as the Centre Georges Pompidou. Most major tourist attractions are a half hour metro ride away - without you needing to change lines. The Gare du Nord is connected to the nearby Republique metro stop on one line, if you come by Eurostar.
The flat is quintessentially French, it really added something to our stay to be able to come back to somewhere with so much character.