Javier's place is great! The room given to the guest has ample space (you have a bed, wardrobe, desk, and some room in between). The location is fantastic; you are walking distance to many of Madrid's main attractions, and there a number of restaurants and grocery stores in his neighborhood. While staying at his place, I walked everywhere; 10-15 minutes to Prado Museum, 15-20 minutes to Thyssen, 5-10 minutes to Reina Sofia; 10 minutes to Plaza Mayor, 15 minutes to the palace, and 15 minutes to Puerta del Sol. There is also a fitness park right next to his place. If you're not into walking, the train/metro station is like a 2 minute walk.
Javier was an extremely helpful and gracious host; he showed me how to use the Madrid train/metro, and provided a map of the city (he circled recommended spots) and a big bottle of water. I definitely recommend his place!