Have been to Koh Samui a couple of times by using Thai Airways and the bus/ferry connection.
Fly into Surat Thani (URT) - buy bus transfer/ferry combo at airport - negotiate for ride in back of pickup truck after arrival on island of Koh Samui for transfer to lodgings.
And everytime have always flown back on Bangkok Airways - lot more convenient and not as exciting. Only excitment is the short runway on Koh Samui. Sort of reminds me of an old aircraft carrier where they have this ramp sort of effect at the end of the runway.