My second stay at this place (previous one three years ago).
I truly recommend this apartment: very clean, well-furnished, very close to the city centre but at the same time away from the bustle of Athens.
Communication with the owner is easy. Costa is very helpful and efficient.
There are bars and restaurants at a stone's throw, as well as a supermarket 300 meters away.
A 2-minute walk to Thiseio tube station (which can take you to the airport in 50 minutes), a 15-minute walk to the Acropolis.
I really can't think of a better place to stay in Athens.