What a great unique place. It definitely has character and we loved our stay here. We had three adults and two kids and there was plenty of room. It has a very unique layout over the 4 levels, but worked really well for us. It was also a heatwave (I believe we had record heat in Paris when we stayed), but the place stayed very cool and the aircon held up well in the bedrooms. The downstairs kitchen was a lovely place to retreat to at the end of a hot day.
The location is fantastic and close to a good sized carrefour supermarket, great restaurants (we ate at Quai 33 just around the corner 4 of the 7 nights), wine bars etc. It also has great access to the metro at Gare de Lyon, Ledru Rollin and Quad de Rapee, which gets you nearly everywhere in Paris or plenty of bus stops close by.
The other fantastic thing was a code entry to the door meaning you didn't have to worry about keys, this was such a great feature for a group and having to coordinate a key as we have had to in the past.
A few things to note (some has been mentioned previously but worth reiterating).
The stairs are quite narrow and winding, so it wouldn't be ideal for people who aren't very mobile. Also the toilets are located half way up the stairs and quite narrow and small, so if you were on the larger side, waist wise (trying to be diplomatic), these would be a little trickier to navigate. That said, we had no problems with them, with my 75 yr old father in law staying on the top floor.
The street is gorgeous as is shown in the photos, with plenty of people photographing and videoing themselves most days. It didn't worry us, my kids thought it funny to wave through the window, however if you preferred complete privacy, this may be a slight turn off. It is only the ground level and if you have the curtains open.
Lastly, the floor boards are a little creaky, so if you have a large group coming and going at different times and light sleepers, this may disturb.
I hope this hasn't come across too negative as we did love the place and I feel these points actually add to the character, however I could see how others wouldn't. We were very sad to leave and would love to return in the future. Thanks Marlene for creating a beautiful home for us to stay in