Hi, Johnna! There are as many different answers for that question as there are people to ask! It really depends on your family and your vacation habits.
I think the best reason to purchase the Disney dining plan is so that you can have a good handle on your vacation food budget. If you know ahead of time that you have already paid for two meals and a snack every day, you don't have to worry about figuring out later on how much money you need to have to cover your meals. It's a great way to plan one more aspect of your vacation that can get away from you if you're not careful!
One drawback to the plan - well, some versions of the plan - is that you really do need to consider making advanced dining reservations. Some people don't like to have to plan in advance where they want dinner or at what time when they are on vacation, but if you want to really utilize the dining plan to your advantage, it's best to make reservations for your table service meals. This insures that you get the dining experiences that you want, as some restaurants do become completely booked.
You can find more information on the Disney Dining Plan
HERE . It gives you the dining plan options, and you will be able to see if you can make one of them work for your family!
Have a wonderful vacation, and whether you decide on the dining plan or not, I hope you enjoy some wonderful Walt Disney World meals!