This is one of the most fiercely debated topics Disney-related!
The
Disney Dining Plan is definitely not for everyone and that's fine. Many, many people, especially families with kids find they simply don't eat enough food to justify the costs so if that's the case, then fine, it's not worth it.
However, if you live for food and and eating in the restaurants is one of the major draws for you to visit Disney World, than like me, you might find you can't do without it. Also, if you happen to really love all the eateries at Disney and having been dining at the sit-dows now since 2005, I know I love them.
In my case, I usually do 3 sit-downs a day and I order multiple apps, drinks, entree and dessert whenever I can, so for me, it's really worth it. It also allows me to pay in advance which I really like to and know I am taken care of for my dining needs (other than the tip and a few extras). Also, very often when I order extra apps, because I have the deluxe dining plan or honestly I don't know why, but I usually don't have to pay for the extra apps somehow. It's a great perk that sometimes happen and they just tell me the computer didn't charge me, and oh well. LOL
Anyway, if you plan on mostly doing counter service and eating fast and on the go and have a shorter vacation, dining plans are probably not worth it for you. But if you plan on alot of sit-down meals and have big eaters in your party, than it would certainly be worth it, in my opinion.