Cristina's apartment is very nice.
When the taxi dropped us off in front of the building, we thought we were at the wrong place. It was just a small, black door on a commercial street. We entered and walked down a mailbox lined corridor into a charming, private courtyard. It is quiet (no street noise).
The apartment efficiency was cozy, and had everything that we needed for a 1-week stay. It was clean. The furniture was comfortable. The internet was very fast and reliable (>200Mbps). Cristina is very friendly and quick to respond to any queries.
There is a grocery store just a few doors away, and numerous restaurants. Transportation is easy. A bus stops on the street outside, and the metro (subway) station is only a block away. They will take you to all Madrid attractions for 1.20E. Again, it is important to note that despite the nearby businesses, it is very quiet once you pass through the courtyard to the apartment.