@bdw - ok I think I downloaded the Alipay app .. not the HK specific one.
Was half English / half Chinese... really could not be bothered to enter my credit card info to give it a try.
Wechat is also a bit confusing with RMB and HKD wallets, but at least its one app and its in english. I got the free $10 by binding my HKD credit card up to it and then got $10 off spending of $25 or more in McDonalds a few times in February.
Now Alipay is doing similar kinds of promotions but they have confusing apps and not in in english for HK market.