I tried to buy a "premium seat" on air asia 3 times. I only succeeded once. The other 2 times the agent told me to "buy it at the gate". I got to the gate and they said "you have to buy it at the ticket counter, or onboard."
They also fill up the airplane in the dumbest way I have ever seen. The fill up rows completely before they start filling another row. The result? Full rows and completely empty rows. The solution? Passengers end up changing seats on their own, much to the consternation of the cabin attendants.
I have flown on them many times because it was convenient for me. Nonstop from where I was to where I want to go. I will never connect on them again. I have done it on two round trips. Giant PITA, and unreliable. Good for non-stops, and cheap. The cheap is not worth it if you have to connect, however.
I think their boarding technique has exacerbated their problem with passengers helping themselves to "premium seats". It is, after all, just another empty row of seats.