Samui's weather patterns are a little different from the rest of Thailand. In April through September, when most of the country has its monsoon, Samui stays fairly dry, but from October to December, it's wet in Samui and drier elsewhere. The driest season of all, though, is January through March.
Ko Samui is all in all a fairly big island. The most popular and commercialised beaches are Chaweng and Lamai, while the northern beaches and their adjacent villages of Mae Nam, Bophut, Bang Rak (Big Buddha) and Choeng Mon are more peaceful choices, and the west coast beaches are still (comparatively) quiet.
A 300 baht surcharge is levied on domestic departures, as well as the 500 baht surcharge on international departures. If you fly via Bangkok to an international destination the tax is only 200 baht as the Bangkok international departure tax is already included in your ticket ex Bangkok. These charges are not included in the ticket price.