No, not really. It's a long, overnight trip and then you've got to connect onto a ferry for a few more hours. Getting a cheap advance purchase fare from Bangkok Airways isn't much more expensive and is far less of a hassle... although if you're on a budget, note that you'll get ripped off by Samui's airport taxi mafia and the steep departure fees.

More info: http://wikitravel.org/en/Koh_Samui