What a wonderful place to stay. The apartment was terrific. Comfortable, well maintained, and quiet. Nice big bed. Kitchen fairly well stocked with basic cooking supplies. Loved the little galleria for breakfast or an afternoon drink (although it was a little cold in January).
The apartment could not have been better located for us-- everything was easily walkable, but it was in a quieter part of town, insulated from Madrid's noisier nightlife centers. Lots of nice restaurants nearby.
Finally, Alvaro was always quick to respond and very helpful. The cleaning crew was also nice (we showed up just as they were finishing). As others have noted, the initial access to the apartment seems a little fussy and unnecessarily complicated, but it worked out fine and once in you get traditional keys and it's easy after that. All in all a wonderful experience in Madrid.