Ageliki's place near Apollonia was lovely. It is an excellently renovated pigeonniere. The house is in great condition, with high-quality finish, and the furnishings are nice.
The location is great - it is quiet and private, and yet a very short walking distance into picturesque Apollonia, which has a lot of restaurants and nightlife, and it is a short drive (typically 20mins) from most places in Sifnos. There is a private parking space right in front of the house.
Also, Ageliki was very responsive and helpful throughout our stay. The place was serviced daily and she brought us Greek treats every time she came by. On one instance we had a small issue at the house, and it was resolved in less than 15 minutes.
One should keep in mind that (1) it is a small place if you're looking to fit more than 2 people; there is a sofa bed in the living room but it will be tight, the space is petite, and (2) the stairs to the upstairs bedroom are steep and might not be friendly to some.
Overall, it was is a great place with a lot of character and a great host. We would highly recommend Ageliki's place on Sifnos.