We stayed for three nights.
The apartment was clean and spacious and if you had the balcony door shut there were no disturbing sound from the noisy street.
The apartment is also perfect situated close to everything and with a movie like view of Acropolis.
It feels secure, you have to use a key card in the elevator and for the apartment.
NS staff were keen to make us feel welcome with email/call about arrivals and cleaning.
The only things that "troubled" us were,
NS should accommodate with a smal ramp at the entry because of the "high" step to the street. We had a person in a wheelchair, it was not a huge problem for us but it could be for others.
Also, the apartment has a thin door and also a gap at the top and bottom, every sound from the corridor is heard inside especially at night. Not a huge problem but if NS could cover those gaps it would surely help with the sound.
We had a very pleasant stay, we highly recommend it.