The good: Location of the apartment is great. Prado museum is literally a block away, and the park is also a block away in the opposite direction. There are at least four restaurants and a coffee shop within a two block radius.
The not-so-good: The apartment was not ready for us when we arrived (after the scheduled check-in time). It was still being cleaned, for which the real estate agent apologized. Not a big deal, we used this time to get a late lunch.
The apartment itself is on a smaller side, especially the second bedroom. The heat radiators are controlled by the building, and heat gets turned off during the night. Though there are individual heaters installed in every room which can be used to supplement the radiators, the instructions said not to run them continuously.
There is no dishwasher, so I assume the dishes get washed by hand by the housekeeper - we found several dirty plates and mugs on the shelve, and had to re-wash them. Also, I have to agree with one of the reviews - the apartment would definitely benefit from having a coat rack or simple hooks installed in the hallway.
The nearest parking garage is about four blocks away, but it was always full. Finding street parking around the building was very hard, especially during the evening hours.
Other than this, we liked the place, and loved the location. It was close enough to all the craziness (Plaza Mayor and nearby streets and squares) to walk to, but far enough to be quiet for a good night sleep.