'Tis true that when flying into Samui, they are the only game in town. There is bad blood between BKK Air & Thai, and they won't be letting Thai land there any time soon (like during my lifetime ~Ώτ).
They are a good airline, no problems whatsoever. You can sometimes find a BKK Air pass that allows multiple hops within Thailand for a good rate. This allows for flights between Chiang Mai to Samui, for example--although not direct ones. When this is available, it's a fine deal.