We stayed only for two days in Madrid and Antonio's place really fit the bill. The location was perfect of our art-focused trip -- we walked to the Prado and the Reina Sofia around the corner. Antonio was an excellent host: he provided us with detailed information on the apartment location and Madrid, in general, prior to arrival and was able to accommodate our late check-in. The only downside of the unit was the lack of access to the elevator (something about the superintendent not sharing the key): hopefully, that's a temporary issue. The unit is about 30 steps up - which is significant for large suitcases.
This is a unit that appears to be used only for rentals (i.e. noone lives there) but as such, it is very thoughtfully furnished with the needs of the traveler in mind: a clean and modern bathroom, effective air conditioning in each room, clean and comfortable bed, lots of hooks, closet space, washing machine, etc. We did not use the kitchen but it seemed to be well equipped.
The unit does not have any exterior facing windows -- all windows open onto courtyards, which means that it's very quiet.
The only tip I would give to future travelers is to not dry clothes in the clothing line in the courtyard (which is accessed through the window), as they may absorb smells of friend pimento peppers :) There is foldable drying rack that you can use indoors.