You could not top the location of Nadja's apartment. It is a five minute walk from the Acropolis, less than ten minutes walk to the Acropolis museum (which you absolutely MUST see) and only a few hundred metres from some great restaurants, including a fantastic Thai restaurant call Tuk Tuk if you need a break from Greek food.
The neighbourhood of Koukaki is delightful - it is super safe, very upmarket and has a lovely village/community feel with lots of locals at the restaurants. Much less touristy than Plaka and Monastiraki which are both walking distance from the apartment anyway.
Nadja's apartment is lovely. Beautifully furnished with lots of ammenities and everything you need to make you feel like you are at home - dishwasher, washing machine, detergent for both, two spotless modern bathrooms, fluffy hotel grade towels and very comfortable king sized beds with aircon and heating in both rooms and the living area.
Nadja's friend Bettina arranged for a taxi transfer from the airport (standard €38 regulated fare) which was very convenient and Bettina greeted us at the apartment. Bettina was wonderful and showed us where the shops, supermarkets, bakeries, restaurants and cafes and attractions were. She was available whenever we had a question.
I highly recommend Nadja's apartment - super high standard. We had a very comfortable and relaxing stay.