I really can't say enough good things about this place.
Javier, the host, effectively acted as our concierge for our trip: he arranged for a taxi to pick us at the airport at 6 am when we arrived from the states (the Madrid metro is great, but I didn't want to navigate it on 3 hours of sleep), and he recommended nearby restaurants and sites of interest that were not previously on our radar. We bought some Real Madrid tickets via Stubhub, which he graciously received for us before we arrived.
Although we arrived at the apartment at 6:45 am local time, Javier was there to greet us, show us the essentials and then quickly left us so we could get some sleep. Note: our reservation started the previous day; normally check in is in the afternoon.
The apartment is located on Plaza Santa Ana, which is in the middle of Madrid with everything from the Prado to the Palacio Real within a 15 minute walk. Also, a central metro station (Sol) is an easy 5-minute walk away.
The inside of the apartment had everything we needed. The bed was VERY comfortable and the shower had plenty of hot water. There is a comfortable common area that we used sparingly (we were out exploring the city all day); there is a kitchen, but we didn't use it. The doors to the balcony overlooking the square are soundproof, so we didn't hear any street noise.
Overall, we had a great stay and strongly recommend Javier's apartment for anyone looking for a central location when visiting Madrid.