I would recommend this apartment (especially when it isn't the hottest part of the year)! Mariano's apartment was a great place for us to stay! My favorite aspect was that it's within these side streets that have really tasty tapas and restaurants. I usually took a nice evening stroll and grabbed a few tapas and cervezas. It's in walking distance some shopping areas and about a block away from the metro red line, which can get easily get you to the palace, Reina Sofia, or the Casa de Campo.
If you are taking classes for Don Quixote, the school is very accessible from their and a has room enough to hang out, study, and cool off.
The only drawbacks were that the apartment smelled strongly of paint and the fan we had was very small. I think we were in Madrid during the hottest part of the summer, so have a good fan and pleasant indoors was very important. I doubt many folks will experience these issues as they seem specific to our time there.
Mariano is also very responsive, so I doubt these will be issues for other guests.