Konstantino's place was beautiful. The scene right off the balcony was unrivaled by any other place we've stayed at in Greece. It was a very nice last night in Sifnos for us. Very relaxing and convenient to the beautiful town of kastros.
I would definitely recommend driving over a scooter to get there because there are many winding roads. Konstantino was very responsive. We had some issues linking to the wifi, which he had fixed within a couple hours. Voula was a very friendly host as well, she was on site at all times to answer any questions.
Honestly, I wouldn't even question it. Go here. :)