(+) Really great location! It's in the heart of Madrid, very close to Plaza Mayor. All major tourist attractions are perfectly within walking distance. And there's plenty of supermarkets nearby (Plaza Tirso de Molina).
(+) Apartment is just like in the photos. No surprises here! Nice furniture and decoration.
(+) Beds are VERY comfortable, including the one inside the cabinet. Consider this a double plus!
(+) Great air-conditioning! Another major plus!
(+) Very fast WiFi connection.
(+) Contrary to our expectations it is really quiet at night. Earplugs not needed!
(+) Check-in was easy, by means of a lockbox located in a nearby (~500m) location.
(+/-) Cleanliness was just OK. Far from perfect but acceptable enough.
(-) One bath towel per person, but only one hand towel per bathroom.
(-) No detergent for the dishwasher (would have used only once, anyway)
CONCLUSION: Despite a few things that get some improvement, all things considered this is still worth an overall 5 (*****) star rating! Will be my first choice for a future stay in Madrid.