I was just "running some numbers" on this same issue. My wife and I are going for three nights next week. The last time we went without kids we did the counter service plan and liked it. Financially, here's what I came up with:
$210.00 QSDP, two adults, 3 nights
-$29.80 Two mugs plus tax
-$36.00 Six snack credits ea., averaging $3 per snack
------
$144.20 Cost for 12 counter service meals
So, I took that number and divided it by the number of meals (2 per night, per person) and came up with $12.02 per combo meal and dessert.
Now, let's take a look at a standard burger combo. For example, if you go to Rosie's at HS (we always seem to end up there at some point) you will pay $9.29 for the burger and fries, $2.19 for a Coke, and $3.59 for a cake. If you paid OOP, that meal would set you back $15.07. In this case, the dining plan "saved" you $3.05.
If you go to Cosmic Ray's at the MK, you can make out even better. A BBQ combo is $14.19, a Coke is $2.19, and (surprise) a cake is $3.59. That meal would cost you $19.97. You just "saved" $7.90 on the QSDP.
OK, let's take this one step more. If we bought that BBQ meal for each of our twelve meal entitlements, we would save $94.00 over the OOP price. That more than covers the cost of the mugs and snacks. In essence, if you choose higher-priced counter service meals, you can make the snacks and mugs "free."
Is it worth it? I think it CAN be, if you choose meals that give you a strong value. I also think you need to be a snack eater. We almost always end up grabbing something light during our time in the park; we know that we would end up buying snacks anyway. Also, are you going to make use of the refillable mugs? If not, you don't get as much bang for the buck.