It just depends as theres many ways to look at it.
The price (assuming you are paying out of pocket for the dining plan) is pretty high. If you want to "beat the house" in terms of menu prices, you will want to go for the higheset price entrée only (usually the steak) or it won't work out in your favor. Or take advantage of the character meals buffets which are like $60+ for an adult- these can give you good value on the plan. If you just want a $15-20 entrée, you are shorting yourself. This became an issue for me, as I felt compelled to order the steak at places where I might have rather maybe ate the chicken or a sandwich or something.
But others, regardless of this, like the plan, because it means their food (outside of tipping) is mostly paid for up front. This can give a comfort factor if you don't want to worry about expenses as much during the trip. I truly do get this, but also keep in mind you are still paying out of pocket for tips, any pre meal soups/salads/appetizers if you like to order those since the plan doesn't cover them, and for any alcoholic beverages.
In my opinion though, I truly believe for most people- if you simply just bring your credit card on the trip and just charge everything you normally care to eat during the trip- you will see the amount spent will be less than the dining plan cost, or at worst, about the same (but I truly think most will see savings). The reason being- You probably will not normally eat all the plan affords you. If you are like us, we eat a quick breakfast in our room to get to the parks as quick as possible. Following this, we don't need a full lunch with dessert followed by a big dinner with desert. We really don't need a dessert at all after lunches or dinners, since there is likely some form of snack had in the park during the day. On the plan, we just found ourselves absolutely gorged with food we were not necessarily hungry for (and we can be pretty big pigs too). We often made it to our "big dinners" not all that hungry and by the time it came to order dessert again, it was like "are you kidding, lol". . Again, at the meal, I might actually prefer to choose a cheaper entrée and not the steak. I might not even want a table service meal every day of the trip.
So, for me, I don't pay for the dining plan. If I can get it free as part of a promotion, I'll certainly take it, but I strongly believe paying for it rarely works in your favor.