bicker had a chart that showed you could come out ahead with the dining plan, even if you never ordered appetizers or dessert. I didn't do that much work (it's a great chart, though, he specified restaurants and prices for certain items

), but I did figure out if you eat one TS meal a day:
$25 meal (that can be a moderately expensive entree alone, or an entree with appetizer and/or dessert, or figure $27 for a buffet since it includes beverage)
$2 beverage
$2 tax
$5 tip
That's $34. For one meal, for one person. Not a cheap meal, but could be a less expensive entree and an appetizer like soup or salad -- nothing extravagant. And that's not an unusually high price for a nice steak, or seafood dinner -- especially at Disney.
The dining plan is $39/day for an adult. So that meal is included, plus dessert if you want, plus a CS meal (because most people don't eat only one meal a day), and a snack worth up to $4 (everybody wants Dole Whip, or school bread, or a Mickey Bar, or something...)
So, if you do one sit down meal a day, then the dining plan is probably worth it. If you don't like to do TS, or don't want to most days, reservations are too much of a hassle, then it isn't worth it. If you did 2 CS meals at an average of $12 each, and a $4 snack, that's only $28, and the DDP isn't worth the money. Also if you are eligible for the DDE card, then that may be a better option for you.