My friend and I had a very nice stay at Roberto's place.
The neighbourhood is quiet and very pleasant with a lot of bars/café/restaurants around. And of course the Retiro is only 5mns away by foot which is very nice.
Roberto's place is very clean and well equipped.
Roberto and his friend are also living in the apartment but they will let you all the privacy you need, but in the same time they will be there for a chat, coffee and any kind of tips you could ask for.
I'd definitely book here again for my next stay in Madrid ;-)