I dont see much on the Agios Sostis forum so felt I must post as have just come back from a lovely week (weather fantastic all week). What a lovely resort, peaceful, laid back and easy to walk to everything.
We stayed in the Lorenzo villas, best villa have every stayed in. Spotlessly clean, everything you need and the owners in the villa on site to help with anything, but not intrusive. One of my fondest memories will be the blue horizon where we had breakfast overlooking the sea every morning, we ate and just didnt want to leave, the view, the sea the little boats, watching people early morning on the beach. We crossed the bridge to the little island (is it called cameo island?), it is beautiful, climb up the steps and although you pay 8 euros, you get a free drink and can sit up there and look at the views of laganas and back to agios. We went to Kalamaki beach (next to the crystal hotel with the man selling fruit sandwhiches!) which was lovely and we drove to Tsvili, beautiful beach, but my favourite place was the "oh so pretty" Agios Sostis. Beautiful island, thumbs up for Zante.