Our tour organizer arranged our flights between Ho Chi Minh City (Saigon), Vietnam and Siem Reap, Camodia on Cambodia Angkor Air. The outgoing flight (#825) was on a relatively new A321, and the flight was smooth and uneventful. Due to the short length of the flight (1:15), the flight attendants served only water to passengers.
The return flight from Siem Reap to Ho Chi Minh City (#824) was on an ATR72, a prop plane that had not been very well-maintained, at least, from the interior point of view. The seat cushion on my assigned seat was ripped all the way around the rear portion of the seat. Fortunately, the seat next to me was unassigned, so I could move; but this lack of maintenance gave me pause about flying the airline, in general.
There are not a lot of choices of airlines that fly between Saigon and Siem Reap, but I would advise checking which plane that Cambodia Angkor Air is using so that you can fly on their A321, if possible.