My wife and I flew Bangkok Airways from Bangkok to Koh Samui in Dec 2000. As USAF aircrew we are pretty skeptical about safety on MANY commuter airlines, and this was no exception. That said, our trip with this carrier was outstanding!
The lounge in Bangkok is clean, beautiful, comfortable, and offers free snacks and email service. The aircraft (an ATR at that time) was clean and appeared well maintained. The cabin crew was professional and attentive...there was even a meal served to all on that ~1hr flight! (and to think US carriers have problems serving beverages in that same time on the same type of aircraft!) The flight was comfortable and did not stress us in the least. AND, as any other Koh Samui visitors can attest, the Koh Samui airport is a real treat! I won't describe it....you have to experience it for yourself.
All in all, we were extremely happy with Bangkok Airways in every regard. Enjoy your trip! (If you need lodging in Samui, I'd recommend the Imperial Samui .... it's not in the heart of town, but is an outstanding property!)