Niki's apartment is really amazing. It's right by the Acropolis and a short walk away from the wonderful neighborhood of Plaka with all the markets, restaurants and bars.
The apartment itself is spacious and the living room + rooftop are perfect to just catch up if you're a larger group of friends. The only (potential) downside is that it's a bit of an uphill walk. This was ok for us, but we were sweating hard by the time we got there.
Niki herself was an amazing host. She responded immediately to all our requests and was very welcoming.
I'd definitely recommend this apartment for those looking to walk around and take in everything beautiful Athens has to offer.