Re folks saying it's been done before, it makes me think of the William Gibson quote "The future's already here, it's just not evenly distributed."
Even if it has been done before (I have no idea) -- if something makes a new tech available to many more people, it's effectively new to them.
In terms of using the card, my wife and I have found it useful. It feels empowering to give the card to a service/person and know you remain in control. Yes, all cards let you do chargebacks, or recover from fraudulent charges, but I don't want to have to fight to recover my money.
I love the feeling of using a number and knowing no other charges can come - that number can never be used again.
This card is empowering.