I had a great stay at Konstantinos's apartment.
It's old-school airbnb - a real person's real apartment, so if you prefer sterile, empty spaces it won't be for you.
It was comfortable, nicely furnished, clean. We didn't cook but the kitchen seemed good and there were plenty of utensils. Balcony overlooking the street (sadly just a view of the CaixaForum but you can't have everything). We stayed over a friday and saturday night and it was quiet at night.
The neighbourhood is the real draw - near so many attractions: the Retiro and botanical gardens, the 3 big museums, as well as plenty of bars, restaurants and cafes and Atocha station if you are getting the train anywhere. We walked everywhere but it's well served by the metro as well.
Konstantinos is a fantastic host - really friendly, genuine, responsive and trustworthy.
I will definitely try to stay here again next time I'm in Madrid and would heartily recommend it to anyone.