A small 2-bedroom apartment in the middle of Madrid. Three subway entrances 1-5 minutes walk away from the front door. Bus stops literally across the street that take you to the great Museums. Great breakfast places nearby (try Chess Coffee). The best kebabs I've had almost across the street (Galaxy Kebab. Cheap & tasty, saved many a night for us.) A few convenience stores nearby. Perfect location, really.
The apartment was clean, Javier was there waiting for us and gave friendly advice for nearby things.
But remember that Madrid never sleeps. It's a noisy city, and the rhythm of life is different than, let's say, northern europe. So you might get noise from the inner courtyard up to 1-2am. If you are a light sleeper, bring earplugs or noise cancelling headphones (I used the latter and slept very well).
Wifi was fast and reliable, beds were large and comfy, everything in the apartment worked very well.
If I travel to Madrid again, I'm definitely considering Javier's apartment again!