We've done it with and without, although the last time we did it was before they changed the
DDP (eliminating app and tip)
I think the dining plan can be great
depending on how you like to eat on vacation.
If you're going to use it to the max and really get your money's worth (we sure did, and then some). In our case it allowed us to try out lots of restaurants at WDW that we never would have if we'd had to pay out of pocket. We kept track, and many days we had TS meals that easily would have cost us $150-$200 just for that meal alone. Pretty good deal when you're paying about $80/day for the plan.
My own feeling is that if you're not going to use it that way - i.e., you've got picky eaters in your group or just aren't into trying new stuff - it
might not be quite as worth it. Although if you're traveling with kids and plan to do some character meals, that could make it worth it right there.
We found that we had MORE than enough food - to the point that the camper fridge was filled with desserts and we had enough QS credits left on the last day to buy lunch for all our friends.
Ultimately we gave up the DDP when the the appetizer/tip were no longer included - and also because we shuffled our budget around to be able to afford an extra week's stay. But that said, we're actually considering doing it again this year - just got to crunch the numbers to see if it will work for us.
My suggestion would be to check out the menus (AllEars and the DIS both have great info) and rough out how you think you'd eat and what it would cost you for an average day and do the math.
Oh, one more consideration. Time. Eating a TS meal every day at WDW will eat up quite a bit of park time and require some planning to get all your ADRs lined up. Your call as to how important that may or may not be.