We spent one week at Alejandro's apartment around New Year's Eve. Our arrival was uncomplicated and everything went well. The room was clean and comfortable, but a little smaller than it appears on the pictures (as was the rest of the apartment). On our arrival Alejandro offered us a lot of information about the city and the neighborhood, which can be very helpful for people who would visit the city the first time.
The apartment in general is well equipped. There is a kitchen guests can use and a small balcony. The host also lives in the apartment so you see him around sometimes. During winter, as it was when we visited, the apartment can get really cold (as temperatures Madrid in general can get as low as 0°C during this time of the year). There is a heater in the guest room that was not always working. The bathroom has no heating at all (or at least it was turned off). So if you're travelling in the winter you should be prepared to dress warmly.
The apartment is located in a quite untouristy neighborhood so you get a taste of authentic Madrid. There are many shops and restaurants nearby. It's not central but has good metro connection with 3 different lines passing by so although you can't walk downtown from there you will get quickly to a lot of places you'd like to visit!
Overall we had a good stay.