I guess it depends how much shopping you do, but I made up for the cost of the card in shopping alone! It's 10% off with a minimum purchase of $50, and it's available at any Disney retail store, including the parks, the hotel shops, and the World of
Disney store.
My suggestion would be, providing you're spending at least 3-4 days there, is to go to the parks first and just look at what you would like to purchase, then go to the World of Disney, and see if they have it there, plus other things that you won't find in the parks. Then you can kind of lump your purchases together to add up to $50 in one store, instead of having $10 here and $15 there, etc.
A few restaurants offer discounts, including some in Downtown Disney, but many of them might be only for lunch. The website also has coupons to print out that might be for a limited time only. For instance, during my October visit, there was a 10% off coupon for Compass Books in DD.
I also saved at least $100 off my hotel package at Grand Californian (thanks to DU!). There's also a 10% discount at the Disney Store, catalog, and I think the website too.
For me, it was well worth it!