Our whole experience was great. We didn't get to meet Kostas himself, but he sent his cousin to meet us at the closest metro station on our arrival in Athens to guide us to the apartment and show us around the area. He recommended a local supermarket and souvlaki shop right around the corner from the apartment. Kostas sent us a list of top things to do in Athens and places to visit which was very thoughtful and accommodating.
Kostas' apartment has an amazing view, impressive during the day and breathtaking at night. The Acropolis is in direct eye line and it was wonderful to sit on the balcony after a long day exploring just over looking the city. The apartment itself is well furnished, very large and comfortable double bed, TVs and aircon units, both a shower and bath. The bathroom has no natural lighting but functions perfectly.
The elevator itself makes some very unsettling sounds but it seemed to work just fine (you have to travel to the 7th floor to reach the apartment).
Our first day we decided to walk to the centre, a 30 minute walk following google maps' most directed route and it took us on the back streets which are about 10 mins from the apartment area and they didn't make us feel too comfortable, so if you decide to walk in the day time I would recommend using the main roads, and definitely take the metro at night.
We had a great stay and would definitely recommend the place to anyone thinking of going to Athens.