First things first, Juan is AMAZING! He will respond to messages faster than your best friend (assuming your best friend is responsive). We booked this trip rather last minute and Juan was very helpful with answering any questions we had regarding the flat, and other logistical items.
We arrived earlier than check-in time on Sunday. Since Juan lives right next door to the flat, he was nice enough to keep our luggage while we grabbed a bite waiting for the flat to be ready. It was actually ready 2 hours before check-in!
The flat is exactly like the photos, no surprises whatsoever. Very clean, tidy and minimally decorated. The kitchen is very well appointed to make simple meals during your stay. There is a washer as well, with a drying rack should you need to do a load of laundry.
Since the flat does not face the street, it is incredibly quiet. If you like to sleep on the earlier side, this would be a good choice since you wont be bothered by the noise outside. The only downfall is you don't have a good street view.
The location could not be any better. You are literally steps from the Mercado San Anton, tons of restaurants, bars and shopping. Nothing was more than a 20 minute walk away. We didn't find the need to use the metro or cabs during our stay which is really nice for your wallets and your waist size!
Madrid is a magical place, we will definitely be back, and we would definitely stay here again!