Our spending money for each of our trips is nothing but gift cards. We've used Visa, MasterCard, and American Express.
The only issue we've had is once the cards get down to about $25 (the experience varies) and you want to use it for a smaller purchase, it might be declined. But we are always prepared for that and take those cards home to use at Wal Mart or Kroger.
But on our last trip, that didn't happen with one of the newer gift cards so I don't really know why it happens. Just be prepared. And always have some cash in your pocket just in case.