I am really torn about how to rate this apartment. If you want to party all night Madrid-style (that is, until 6 or 7am) or you can sleep through anything, you probably can't find a better place. If you want to sleep between the hours of 1am and 6am, you should completely avoid this listing (I mean it--read no further so you don't get seduced by the positives).
Overall, the apartment is really excellent (other than the noise problem).
Positives: Liener was great--communicative, accommodating, easy to work with; check-in and check-out were simple; the apartment is safe, clean and modern, and includes a dishwasher, washer and dryer; the location is amazing--cool neighborhood in easy walking distance to the major museums, Retiro Park, Plaza Mayor, Rastro and so many other sites/attractions/restaurants.
Neutral or quirky: 3 flights up with no elevator; the bathroom door is simply an opaque sliding glass panel that doesn't seal smell or sound; the bathroom also has an opening high above the toilet that vents directly into the kitchen (if you have modesty issues, probably not the best experience).
Negatives: NOISE--there is a nightclub just below the apartment that ramps up around 1-2am and doesn't stop until 6-7am. It is so constantly loud in the apartment it almost feels like you are hosting the party. We bought earplugs but still couldn't sleep because of the constant excessive noise, and this was in the middle of winter; there is a periodic faint smoke smell in the closet area of the back bedroom; WiFi doesn't work well in the back bedroom.