We stayed for 3 nights and enjoyed our stay.
The flat is really near Atocha station - 7 minutes walk which was helpful. It was then about 30 minutes walk to the more touristy hot spots. Although lots of bars and restaurants near the flat.
The flat is nicely decorated - very modern. Wifi worked well and kitchen was well equipped.
We didn't meet Cesar but communications were good and check in/out hassle free. All in all a great little find in Madrid!