Hi GI Jane,
I haven't used one of these cards, but they have been discussed on the message boards. They are getting mixed reviews depending on location. One traveller on our America's Parks & Canyons tour took one of these cards and had no problems using it.
I would suggest that even if you take one of these cards, it is always worth travelling with a major credit card, as these are widely accepted throughout the USA, and some US Dollars in cash.
Kind regards
Julia, your editor