As others have mentioned they take them at any restaurant (in the parks and hotels, not downtown disney) that takes credit cards. Keep in mind thought that you cannot put tips on them so if you want to tip you will need some extra cash. We used them for pretty much all of our purchases our last trip, including special meals like the WOC dining package with no problems.
I mentioned this in another thread, but if you have a Target Red Card you can buy them at target for 5% off. Also if you have a Sam's Club membership I have seen them for sale there for close to 5% off as well. They are a great way to save some money.