as somebody who has used the dinning plan 6 times, I can tell you the only time it's "worth" it, is when it's free.
The plans are the best value specifically for families who have children under 9, that like to do the character meals. In that aspect, then yes, they may be of some value. After paying for the kids character meal, you are existentially getting a kids meal and snack free thereafter.
In order to break-even or even nearly come out ahead, you really have to plan your meals out.
You basically have to find and plan out to eat the most expensive meals you can find, even if they are not the ones you want, in order to make it worth it.
But, for an adult, the value really isn't there.
Think about it, the adult "regular" is approx $60 a day.
Break it down, that means your table service would have to be approx $40, your counter service approx $15 and your snack at least $5, now obviously these numbers could fluctuate either way, every day.
You'd be hard pressed to find a regular table service that rings in at least $40 for the entree, sides and a soft drink. The most expensive entree at AK is the steak at Rainforest Cafe comes in at $31, unless you're drinking a coke that is $9 or more, you haven't even broke even. H/S has a $30 steak at MM, again unless you're drink is $10+, you paid more for the plan, than you were able to use.
The less your table service costs, the harder it is to use the left over value through the counter service and snack credits.
I personally assign no value to the refillable mug. The cost of the mug and refills is essentially absorbed back into the plan, for all those thousands of people who don't utilize the full amount they have pre-paid. Okay, the mug may cost Disney $1, but the soda come in at .2-.3 cents per fill.