We didn't meet Charles because he was away from Madrid at the time, but his sister was very kind to welcome us and provide us with all the necessary. The room was spacious and comfortable enough, and everything looked clean and organized, including the toilet. As mentioned by others here, the location couldn't be better: it's less than 5 minutes walk from the Puerta del Sol and ridiculously close to a metro station... most of our itinerary was, however, possible on foot.
The only thing you should take into consideration is that since the room is located in a very busy and crowded area, it can be very noisy... we didn't have problems with that, closing the window helped a lot to reduce external noise .