The dining plan will definitely save you significant money if you do it right. By doing it right, I mean, don't eat at all the 20 dollar buffet places, or get the 16 dollar entrees and expect to save much on the dining plan. The dining plan saves you money if you plan to eat the more expensive items.
Now, whether it is worth it depends on how you like to do your vacation. My wife and I like to do several things off property, so after doing the dining plan twice, and saving money by doing it, I have decided we won't be doing it again. The reason why is because I don't like having to always be keeping track of how many credits we have and having to figure out in my mind how to use them right so they don't get wasted. Since we like to go off property, if we go to Universal, for example, obviously we're going to be eating at least one meal there and paying out of pocket. So now I have to plan to use that extra credit somewhere else during the trip so it doesn't get wasted. So, for me, all that hassle isn't worth it anymore. Next time, we're not doing the dining plan, and we're just gonna eat wherever we choose, whether on property or off, without having to give any mind to making sure we use our credits wisely. If we're in Kissimmee for some mini-golf and decide we want to eat at IHOP, we can be free to do so without feeling like we have to eat back on property to get our money's worth on the dining plan.
So, it can definitely save you money, but there are other things to consider other than money in deciding whether it is "worth it". Do you want to have the freedom to eat wherever you want, whether on or off property, or do you not mind being "tied down" (for lack of a better term) to having to eat 1)on property and 2) at 1 table service and 1 counter service per day?