We stayed there for 4 nights during an extended weekend visiting Madrid.
The place was really nice and convenient, in the beautiful and animated Chueca neighborhood. It's close to many good tapas restaurants and bars (we recommend the Trinity tavern just in front of the flat, where we got a warm welcome). The flat is from walking distance to the hyper center (gran via, Sol) , but also the museums, or the Palacio Real if you don't mind walking about 20mins.
We enjoyed the flat, really clean and functional. Mind that it can get a bit noisy in the street during the night, as there are bars in the street. We ended up sleeping rather well however (the bed is great).
Nicolas is a great host, efficient, very welcoming and flexible.