Yialós or sometimes just Symi, is a beautiful harbor. While it is well protected from the meltemi (the Etesians) (the Etesians), it can become problematic with strong South winds. It is also a busy harbor and subject to considerable ferry wash. It is not an easy harbor for yachts because it is deep and its bottom of mud and rock does not always provide good holding.
It is best to moor side-to, if there is room, although this is unlikely in busy periods. Otherwise drop the anchor in around 15-20 m and after testing its holding come stern-to. Stay clear of the ferry and hydrofoil area. There are several good restaurants and provision for shopping.