Your mileage will vary, but we wouldn't dream of going to WDW without the dining plan.
Beware: it's more food than you will eat. If you trip is more than a couple of days, you may find yourself giving chocolate cakes to other people's kids by the time the trip is up!
The standard dining plan gives you one table service meal, one counter service meal, and one snack per day, at $46-$48 per day (depending on time of year).
If you add up the cost of a typical snack, typical counter service meal, and a typical table service meal, you'll find that it's cheaper (not terribly so, but cheaper), to use the dining plan. Of course, appetizers are not covered on the dining plan (which you might want), and desserts are always indcluded (which you might not want). It is certainly possible to dine for less than the dinign plan costs.
For us, the real value comes in the flexibility - if we want to order the most expensive thing on the menu, we don't have to worry about the cost. If we want to trade in two credits for a character meal or a show (such as the Luau at the Polynesian), we can do that.
We like to combine the dining plan with the reusable mugs - we eat a lot of counter service dinners at our resort, use the mugs for the beverage, then carry our bottled 20 ounce drinks (from the dining plan) with us to the park the next day.
Again, your mileage may vary. If you do a lot of table service meals, you might want to look into the "Tables in Wonderland" discount card - a $60 fee gives you 20% off of tables service meals, including wine (not covered on the dining plan). However, TiW gives you nothing off of counter service or snacks.