I love Italy but I seem to be the only person who doesn't like the area around Sorrento (Amalfi, Capri etc). My husband knows it like the back of his hand as he took school groups there for over 20 years in a row, and loves it.
We went to Southern Italy one year in September (the other side: Puglia) and whilst it was great and the weather was just right, the beaches were filthy and many hotels and restaurants were already closed for the season. Cilento may well be like that, too, as it's not such a tourist area as the Gulf of Sorrento.
So, perfect in September, but don't expect the facilities you would have in the holiday season.