We had a wonderful time in Madrid and Carlos&Frank's apartment was ideal for us! Very good location right in the city center, we spent hardly anything on Metro tickets since we could walk to most places.
The place was just like in the pictures, fully equipped with a small kitchen. It is also very quiet at night as the windows open to a light shaft, so no noisy street to disturb your sleep.
Carlos met us when we arrived and explained everything perfectly. He was even so kind to order a taxi for our day of departure (as we needed to leave extremely early in the morning). Everything went smoothly and we highly recommend Carlos&Frank's apartment.