Excellent all around as reviews indicate.
Konstantinos is a great host, always available before, during, and after the stay. He arranged for Panos to drive us from and to the airport. In our drive back to the airport Panos took us to the Temple of Poseidon at Sounion, about an hour drive from Athens. Absolutely recommended. Think of it as a small Parthenon on top of a hill overlooking the deep blue Aegean sea, is beautifully dramatic.
One issue we did have was an extremely strong cigarette smoke odor in the kitchen, especially at night. The smell seems to come from one of the neighbors. We basically did not use the kitchen at all, kept the door closed all times and covered the floor with a towel. Having said that, the cigarette issue is bad for sensitive folks but it would not stop me from staying in the apartment again as the pros completely outweigh this inconvenience.
Restaurants right below the apartment are touristy but really good. Especially LIONDI serves really fresh octopus and the crew is extremely cheerful.
One final recommendation: go to the Areopagus Hill at sunrise, is a 10 minute walk from the apartment. At the footsteps of the Acropolis, you get a 360 degree view of the city. Perfect picture spot.