There's a ton of threads over on the Disney Restaurants board about the Dining Plan, including a pretty thorough FAQ that you should read if considering it.
We did it in January and loved it. Our table service bills were often ~$50 or so without tip by themselves (for each person), so it was like getting counter service and a snack for free, plus saving money on the table service. We didn't use all the snacks (I think that's a common issue), but it was nice having the option - but we usually used the snack credits for water, etc.
We did usually end up buying some form of breakfast out of pocket, occasionally using a snack credit for a piece of fruit or whatever.
Of course, we ate more at the table services than we would have if we weren't on the plan... but that was part of the fun.
Alcoholic drinks aren't covered, so if you plan on drinking at most meals (and especially if you rarely do counter service), the DDE is probably a better option.
However, probably one of the best parts of the Dining Plan is how easy it makes it... on our trip, with six adults, it was guaranteed that if you had a regular bill (even with a DDE discount), that we'd spend 5+ minutes figuring out who ordered what, calculators would come out, tips would be figured out, and someone would probably end up paying more than their share. With the Dining Plan, we ordered anything we wanted (ignoring the prices) and when done, just signed the bill. No dickering, no discussion, no worrying about the tip, no lingering resentment because you just know that you had to pay more than your fair share of the tax/tip.
Even if it didn't save a penny, it would have been worth it to us for that alone!