BEAUTIFUL. This flat is absolutely gorgeous. The location is very central to many things to visit in Madrid--in fact, the Museo Reina Sofia is just across the plaza and has free admission most evenings between 7-9pm. It's also very close to Retiro Park & the Prado. The area just outside the building is very busy, but we didn't have any trouble with noise inside the flat.
Luke, one of the managers of the property, greeted us upon check-in and gave us the tour. He and Roque also run a wine shop, and we stopped by there for some recommendations (both for food & wine). They were both exceptionally helpful.
Be aware that there is not an elevator to get to the 2nd floor...we had a lot of heavy luggage that we had to carry up the stairs, but it was totally worth it for this flat. We really enjoyed our stay.