We used to - until they changed the dining plan (eliminating appetizer and tip) and then it just wasn't worth it for the way we like to dine on vacation. Back then we got all our money's worth and more out of it - and ate at lots of places we never would have paying OOP.
The last time we did it (2007?) we started to feel like we were planning our days around dining, which started to not be fun. Plus it was WAY more food than we could eat (and we're good eaters) and we ended up with a fridge full of desserts in the camper.
When we started making ADRs and dining plans this year, we looked at it again to see if it would actually save us some money. I whipped up a spreadsheet and went through the menus online and actually compared apples-to-apples.
Gotta say I was surprised when I added things up and found that for 3 weeks (incl. more than a few signature dining locales) we would save more than $300 by NOT getting the DDP.
And we don't need to plan our days around dining.
I'd encourage people to actually crunch the numbers based on how you eat and like to tour and see if it's worth it for you.