No issues, decent exchange rate. Have a look through this thread for more information.
I used gift cards to pay for a dining plan (through DVC) the other day. Worked out great, since I was pretending to be my mother on the phone (long story) and didn't want to have to use her credit card to pay. So I bought CAD $1500 in gift cards from Shoppers with my own credit card and used those instead.
The USD balance fluctuates daily with changing exchange rates, so if you want or need to know exactly how much remains on the card, you'll need to be prepared to keep an eye on it.
I bought the Gift Cards from Safeway. Had no problem using them in WDW. They are in Canadian dollars and when you use them they give you the exchange rate of the day. So you save the 2.5% forex.