We loved our stay near Platis Gialos. The owner was very kind and gave us a lift into town as he was going there. He also did a bag of washing for us and it came back the next day dry and clean for us. There was homemade marmalade in the fridge which was delicious.
The apartment was right by a bus stop which was really convenient. We travelled to Kastro and Platis Gialos and Appolonia all by bus.
We would recommend this place for someone wanting to stay in Sifnos.