The apartment is nice and modern. As far as I can tell, it has been recently (may 2017) renewed. Although the street (Zinonos) is quite busy at daytime and at night, the windows kept the place isolated. The apartment has a washing machine, air conditioning and dish-washer. The shower is amazing.
Nestor is a great host. He was very accommodating (he let me arrive early and leave late) and was always available when I texted him.
I stayed here for work, but I can recommend it as well for a vacation. Omonia station is 2 min away and you can reach some sightseeing spots (National Museum, Monastiraki, Sintagma) easily by foot.