We had a great stay in Kostas' apartement. It is as spacious and well equipped as described and the location is great as well. It's walking distance from the Acropolis for instance, and as Athens has a compact city centre most places of interest can be reached by foot or with a short and cheap metro (stop is around the corner) or taxi ride (traffic is terrible though, so keep that in mind when taking a taxi).
Furthermore, Kostas is an excellent host. He was easy to communicate with, responded quickly to our messages, was very kind and made sure we had all we needed to enjoy our stay.
I can highly recommend this apartment.