I lived in Bangkok the last two years and took PG often around Thailand (USM, KBV, HKT, CNX, TDX)...never took them internationally.
Usually very pleasant experiences: efficient and friendly FAs, decent food that is free, no fee for baggage, great safety record and reputation amongst locals. Planes are definitely on the older side...at least the interiors feel that way - lots of wear and tear. But, for a short 60 minute hop, it isn't a big deal.
USM is a bit frustrating on departure - check-in is in the middle of the complex but depending on destination, gates can be all the way at the far ends. As the terminal is open air and Samui is hot and very humid, no matter the time of year, this can be a real drag. On arrival, you definitely feel like you are landing on a tropical island, and baggage reclaim is pretty fast. Car rental is usually a breeze, though return can be a challenge if it is later in the evening or earlier in the morning as the kiosks are not always manned when flights are arriving/departing.
Blue Ribbon Lounge at BKK is very basic, but decent.
Enjoy your trip!