We spent five nights at Sofia's place and overall we can recommend it to anyone who considers visiting Serifos.
The housekeeper, Mrs Sofia was very friendly and helpful. She met us at the port and was extremely helpful pointing out what to do and where to go.
The apartment it's in reality nicer than in the pictures, which weren't bad at all to start with. It is also practical, well equipped and in walking distance to all restaurants, bars, mini market etc.
Overall, I highly recommended this place for a simple and pleasant stay in Serifos.