This is an excellent apartment, especially for a larger group (we were 7). It is in a classic "old-school" building in a very nice section of Madrid. The location is perfect for the Prado, Puerta del Sol, Plaza Mayor, Royal Palace, and Retiro Park. The area is full of excellent restaurants and bars. The bedrooms are extremely comfortable and the apartment was very quiet at night (no street noise etc.)
The host Mar is great! She was extremely easy to get in touch with, was waiting for us outside the apartment when we arrived and gave us an excellent overview of the area with multiple great recommendations.
The only minor negatives -- the common area is a little small for a larger group. The kitchen is in the very back of the apartment near the bedrooms and down a long hall from the dining area/common room. The showers are somewhat odd (at least from a USA perspective) -- quite small, and actually somewhat of a complicated set of buttons to control various jets.
Would highly recommend.