We've been to WDW many times - both with and without the
DDP and TiW.
A few years ago, it was worthwhile. the price was such that if you basically followed the plan, you saved a bit of money. Now it is priced so that it's hard to break even. The only exception is kids under ten.
Prior to DDP we took a trip with extended family where part of the family grumbled about the cost of every TS meal. Getting DDP initially seemed like a solution. My family tends to order low cost items on the menu, avoid all desserts, drinks only water. Initially DDP meant we could splurge. In practice though - if you splurge- the cost of your tipping goes up. It just can't be said that tips are the same both ways. Not for us.
On subsequent trips...we found we had just WAY too many DDP credits. The TS desserts are fun, but the quick serve ones are often kind of lame. Much of my family actually LIKES drinking water with our meals. We also skipped/tossed many desserts.
Another downside is that there's a -relearning process every trip. You have to follow the rules when you order. You can't just walk up to a QS and order an entrée, then later get dessert and your beverage some other place. Also, "a snack" credit can be used for a $1.79 soda, a $1.29 juice box, or a $3.99 ice cream sandwich. If you use all your snack credits on $1.79 sodas, you'll probably lose money.
You are also locked into all your meals. If someone isn't feeling well, or just isn't hungry - no $ back.
The TiW discount likely won't blow you away either. TS meals at WDW are kind of crazy expensive. It's part of the overall experience of WDW, I suppose. It helps, and does include almost anything - even adult beverages and appetizers. But it is not free. Now that it costs, I think, $100? That's a high break even point- especially if you ALSO need to buy an AP. (AP's have a few dining discounts by themselves)
I tried one trip where we had TiW, AND DDP for PART of the group. (12 people). HUGE mistake! It was WAY too confusing for everyone! The servers hated us!
Ultimately, we've come back to thinking TiW is the best deal. Or no discount at all. (to get TiW, you often have to get an AP.) If you aren't on the DDP, you are free to order what ever you want.
It is also worth mentioning one more option. Concierge level! It's not exactly a discount plan, or a complete food plan, but if you are staying deluxe, and meet certain criteria (4people per room), we find it about even to the other options with added convenience. Concierge = a quick continental breakfast, soft drinks, beer/wine upon request most of the day, snacks, appetizers 5-7pm, and desserts 8-10pm.