I've been quite successful with bidding on AirAsiaX, both to Premium and to Spare Seats (not offered anymore)
[URL unfurl="true"]https://upgrade.plusgrade.com/offer/AirAsia[/URL]
That said more recently I've found Scoot is often cheaper.
The hard product is great, albeit I prefer the soft product (food, drink, service) with Jetstar Biz
