We had a great time in Kythnos and the owners were absolutely great, polite, and very communicative! The apartment is new and equipped with nice amenities and details.
However, I believe the apartment is overpriced. Reasons: 1) Poor natural ventilation/lighting (windows only on one side of the apartment - the narrow one) - we had to turn on air conditioning during night even though outside was constantly windy and chilly; 2) floor cleanliness issues (while the apartment was cleaned before check-in, the floor is made of porous cement and dust gets trapped inside the pores); 3) location is okay but not something that justifies the price (view during night is blocked from a public street light). For the same price, we found an old, stone-built, 2-floor, whole house inside the castle area of Serifos, same season, with a gorgeous view and great natural ventilation and lighting, and Serifos is generally more expensive than Kythnos. In my opinion, the price should decrease by 20-30%. That would make the deal fair.