Here is the test that I usually use:
1. If you have 2 kids or more and plan on doing a few character meals the plan may be worth it. The cost of 1 character meal is usually more than the cost of the child's plan for the entire day.
2. If you like having steaks at dinner, the plan may be worth it. Steaks are usually the high ticket items and a steak dinner with drink and dessert for an adult can also add to your value for using the plan.
3. If you research the menus and what is available for snack credits, you can really max out on the value and save a lot of money. This does take some time to figure out where the high ticket items are though. List of menus and prices:
http://allears.net/menu/menus.htm
There is a sticky posted with a list of what you can get with snack credits (there are hundreds of things).
With the dining plan, you will at least break even I would think. But, you might be ordering things like desserts that you might not necessarily. However, most people like the dining plan as a budget tool to know how much they are spending in advance.
I have tried the plans and I find that they are a lot of food and don't really suit my eating style. However, I did get value out of them and it gave me an excuse to try more high ticket things ...and a lot of desserts.
Oh, and if you have leftover credits, you can't get anything refunded. Use any extra credits to get snacks or take out meals at the food courts or earl of sandwich or wolfgang puck express for your trip home.