Got booked on a Thai Lion flight. Was a bit apprehensive seeing the reviews but the flight was fine. No frills from CNX to DMK on a new 737-900. It was a cheap flight and we needed to buy extra luggage. As the reviews said we tried to pay with a US issued credit card and it wouldn't work. Our option was paying per overweight kilo @ 400BHT per kilo or truing to pay at the airport. Since it was way cheaper to pay at least 4 hours hours before the flight we decided to jump in a grab the night before and head to the airport office. Luckily it was still open and we paid our extra luggage (they don't take US issued credit cards either so make sure you have plenty of baht. And make sure you get a receipt. We suspected that we might have "issues" at the airport when we checked in and we did so we showed the receipt and it was sorted out.
In-flight service is fine, well, they don't serve anything unless you pre-ordered but the flight attendants were nice. Seats ok.
Overall it got us from CNX to DMK no problem, no delays, and all our luggage made it ok. Would I take them again? I probably would if they were still on the cheap end!