Glad you enjoyed Corfu and Kassiopi. As other have said, another convert!
May is a lovely time to visit as the island is still very fresh and green for the winter rains (yes, I know, it has rained during the summer too!)and all of the spring flowers will be out. Also. from the middle of the month you should be able to see the fireflies at night - dark areas in the olive groves are a good place to spot them.
The only thing about May is that, early in month, you might get some rain and it can be cool in the evenings.