I've only used it once (only flown with Air Asia once), but I wouldn't try it again.
We applied for the flat beds from KUL to OOL earlier this year, and were advised at checkin there were no seats and we weren't successful. I gave it two weeks after landing and then not hearing anything before I called the Optiontown people asking about my refund. When I did, they said that they don't process any refund until the customers actively phone in (in direct contravention of their own policy which states they'll process refunds within a week if unsuccessful), and that because one of us had been upgraded we'd only get half back.
Neither were upgraded and we still had our sardine class boarding passes to prove it, but they maintained that Air Asia had told them we'd been upgraded. I called Air Asia directly and confirmed that their records showed we weren't upgraded, but Optiontown would have none of it. I wasn't until a month of aggressive calling later and after I'd told them that I was initiating a charge back the next business day that I finally got anywhere.
I know others have had success, but I'd be making sure that you're prepared to fight for your money when things go wrong before booking.