We spent 3 weeks in Alvaro's apartment. I was in Madrid for work and my husband accompanied me for sightseeing. Check-in was easy (Alvaro waited for us in the apartment), as was check-out. The location was perfect: close to my workplace and close to the Prado, Reina Sofia, botanical gardens, and Thyssen-Bornemisza, which were on our must-visit list. The bus stops across the street have many lines, and the metro stops (Artes and Palos de la Frontera) are very convenient to get around Madrid. Despite being on a busy street, the apartment is surprisingly quiet. The bedrooms are completely shielded from any road and hallway noise.
The air conditioning units in the living room and the bigger bedroom work well (and thank goodness, because we arrived in the midst of a heat wave, with temps above 100F). The hot water in the shower was plentiful and the bathroom vanity had enough room and storage. There are a lot of outlets throughout the apartment, so you don't need to bring your long extension cord. Also, this may sound odd to note, but there are a lot (30+) of hangers in the closets, which was great for the length and nature of our trip.
The neighborhood has many grocery stores, fruit stands, and bakeries, making it easy for those who want to make breakfast (or cook). There is a large Moka pot to make coffee.
I can't think of anything negative to say. Note that if you have mobility issues, there is one small step separating the kitchen area from the bedroom hallway and another small step to the living room.