Just to echo the many other voices: Placed an order today; a couple of hours later, received an order saying that the order was cancelled. No reason given. Called customer support: they also couldn't give me a reason other than "the system" didn't approve the order.
UPDATE: After an initially poor experience, I've been using GiftCardMall for nearly a year now with no further issues. Sometimes shipping is a little slow, but on the whole the gift cards work without any problems. Upgrading from one to three stars.