This is great choice for a short stay in Madrid. The location of the apartment is unbeatable, very centric and close to the action but in a quiet street. The size and comfortableness is nice if you are 2-3 guests. Might be a bit tight for 4, but still doable. Beware that there's no elevator, and the stairs can be a challenge after 6h of eating tapas in Madrid, but it's doable and again, you just ate a LOT so maybe it's a plus.
Juan was a great host, and kudos for a very good internet connection!