There are a lot of people with different opinions on the subject of the dining plan being worth it. In the end there are really two ways to look at it; you can do the math and make a factual determination or you can decide if there is value in not having to think about the cost of what you're ordering.
Going the factual route you would need to decide where you wanted to eat and at least narrow down the options of what you would like to have at each place. Will you actually get the included dessert and alcoholic drink or will you pass on those? Normally my wife and I try to be pretty reasonable but when we're on vacation we like to treat ourselves and so we do get dessert every night, and you better believe we take advantage of the alcoholic drink. For the two of us we have found that we usually come out slightly ahead when using the dining plan, and while we sometimes will, we don't always get the most expensive item on the menu. Another point of added value for us is that we often go in March during Flower and Garden, and we enjoy using our snack credits at the different food booths. I definitely consider those a better use of a snack credit than an ice cream bar or a bottle of water. However, once our daughter turns 3 and would be required to get the plan too we're less certain on that. She actually has a good appetite but I don't know that she's physically capable of eating enough to justify the cost of even a child's dining plan.
But then there is the more subjective idea of worth. One thing that some people like about the dining plan is, whether or not they get actual cost value from it, the feeling of the trip being more "all inclusive" and, if they decide to splurge some nights but not all, they don't second guess themselves because they are thinking about the out of pocket cost. I, and most people that would think that way, are aware that they may not be coming out ahead in terms of actual cost but it just removes that sense of hesitation around ordering the more expensive items on the menu if they want to. That is entirely subjective, though.
DW and I have loved having the dining plan on several trips now. In our own personal experience we do not find ourselves eating too much. We also don't mind taking the time for the table service meals. For us it's nice to get off of our feet for an hour or so, talk about what we've enjoyed about our day to that point, and come up with an idea of what we still want to do with the rest of our day.
You will inevitably get people that say that it is 100% not worth it and you'll get some that say that they wouldn't take a trip without it, and in my opinion they are all right. They have all either done the math or taken the time to consider the subjective value (or both) and made the decision that is best for them. I am telling you that to date my family has loved having it, but we are cognizant of the fact that the financial aspect of the picture will change soon as our daughter gets old enough to require the plan and we will reevaluate at that time, looking at both the cost and the perceived benefit.