We flew Bangkok Air from BKK to Samui in March (having done the LHR-BKK bit on BA). as they are the only way to fly to Samui. They were absolutly fine. On time, (unlike BA which was 90mins late so we missed out connection and had to get the next one) and efficient. Free cold drinks in the lounge by the gate very welcome after an 800m dash between the international and domestic terminals. Aircraft (ATR's) were fairly new and clean. Samui (USM) airport is beautiful. Thatched huts surrounded by flowers. Full of backpackers.
Good luck on Garuda!