We struggled with this.....a fun struggle though!
We decided not to the plan for the following reasons and its really the combination of them all instead of each one in particular if you know what I mean:

We are four adults because my children are 14 and 18. My 18 year old DD does not eat like an adult.

We are BIG appetizer eaters. In fact sometimes DD and I only eat the appetizer and skip the main meal.

We SHARE counter service. Many times we order three counter service meals and share them with each other. On the rare occasion that we are still hungry we simply order one more or another order of chicken fingers or fries...you get the idea.

We do love TS. Out of our 11 night stay we have 8 choices that we will get ADR's for. HOWEVER....two of them are NOT on the dining plan...Rainforest Cafe and TREX.

We don't like to get dessert at the TS location. Usually we are so full that we get dessert some other time in the evening.....like we LOVE stopping at Ghiradelli's for an icecream treat when we are in DTD.

I actually went through each menu and added things up and found myself ordering things just because I was ENTITLED to them instead of what I really would have done. In addition we found ourselves looking for the extra three TS choices to fit into the plan. And without spending too much time on this....eating more then I need to eat or would eat is not a good idea for me

Refillable mugs are not on the regular dining plan. Upgraded plan with three meals is just way too much at $70 per day per person...For our 11 night trip that's THREE THOUSAND DOLLARS...I know I can spend less then this.

We are SNACKERS. Usually we end up "grazing" through the day. One thing we LOVE to do is "eat around the worldshowcase" We always have one day when we sample something (sharing is involved) at each of the countries....also yogurt or bananas/fruit/bagels for breakfast too.....watching a parade....get some popcorn....wait look at those dole whips.....yipeee there are the mickey pops.....you get my point!

11 nights was just way too many TS for us to even think about.

It is TRUE if I altered the way we usually do things and stuck to the basic plan ... $40/day per person is really not much money. In fact, I could never take care of all of us for $160 per day. BUT typically, and we went last Easter...we spend about $200 per day as an AVERAGE. And I know this for a fact because I divide the food money into envelopes $200 for each day....if we have left over we add it to the next day, etc. and we didn't run out. When I add a 20% tip on a $100 (low side) TS dinner...I get $20 so that cuts the savings down to $20 and if I add the extra snacks and appetizers I will need to buy anyway.....I come out spending more money.

We always do ADR's but we also like FLEXIBILITY. Usually this goes back to the snacking issue.
So for us....it didn't seem to be a good fit.....but like so many others have said....its about your dining style and what your family likes. I have read many many posts when I was doing my research and it seems that if you have children the plan really pays because their cost is so small compared to the adult price.
I hope I helped.