I spent several weeks in this apartment and had a pleasant experience. Sergio and Elena were very attentive whenever issues arose with the apartment. This apartment is very centrally located and it would be especially good for a young person or a couple wanting to be "in the middle of all the action" at all times, including the crowded streets of central Madrid, the lively bars of La Latina, and the very multicultural neighborhood of Lavapies. Basically everything in Madrid except Madrid-Chamartin Railway Station is within a maximum 25 minute walk from this apartment.
The apartment is furnished with what seem like basic items from Ikea. It is located one floor above street level, which is nice as it allows one to feel connected to the street while being safely above it. The apartment's floor-to-ceiling windows open in a way that almost makes it seem like the apartment has a balcony. It is not a luxurious place, but everything works and it is private. There is an elevator, which people with heavy bags will find helpful.
Unfortunately, the wifi was a real problem in this apartment. The wifi worked very badly for much of my stay. Finally, the wifi stopped working and I had to complain, at which point I was given the password for a different wifi network that actually did work.
Pros: Great location for people who want to be around lots of action. Very good Metro access. Sergio and Elena were very attentive hosts.
Cons: Bad wifi, occasional smell from pipes in bathroom.