Niko and Christina's place is located just across the street from a subway stop AND a taxi station. We were able to get checked in and walk around the area finding a local sports pub to watch soccer, grab some local food, and a coffee in the morning before heading out to view the Acropolis.
You can walk to both the Acropolis and the Panatheniac Stadium with no problems. We took the train from the Airport and decided on catching a cab to the Ferry port of Piraeus early in the morning. Both options were very easy to catch just outside the apartment.
The apartment itself is very clean and is as described. We had a great stay in Athens.