We stayed at the apartment in Kolonaki for four days in September and loved our time there. The apartment itself is very clean, spacious (easily fitting our family of three), well-appointed and maintained. It has all the amenities necessary including air-conditioning to help with the summer heat. The team at Upstreet were also very good at communicating and made the stay very easy.
However I would say the best thing about it is the location; it is brilliantly situated in the lovely suburb of Kolonaki on a street filled with cafes and bars, which were buzzing all evening. There are great restaurants nearby and the funicular up Mt.Lycabettus is very close by.