On the positive side, the apartment is well located for public transportation, with two bedrooms, a large dining/living room area and an adequate size kitchen.
However, the small bedroom seems to be adjacent to the kitchen/dining area of another apartment and the soundproofing is totally inadequate: you could follow the conversation the people were having as if you were in their apartment. In fact, we were awakened at least three nights during our 8 nights stay, at various times (midnight, 3 AM) by the occupants of the other apartment, either dining and conversing or cooking or cleaning the kitchen. The other bedroom is OK, but since we were 4, we needed both bedrooms. I suppose it must be worse in the summer, with the windows opened.
As for heating, there is heated water radiators in each room, but the central heating of the water seems to be stopped at night. So, in the morning (we were there in the winter) the apartment was cooler (not cold, but cooler). There is a space heater that we used to kill the chill, but if anything else was used at the same time (hair dryer, electric plate, washing machine), the circuit breaker would trip.
These things did not prevent us from enjoying our stay in Madrid but if we were to return to Madrid in the winter, we would pay more attention to these two points.