If you are staying at Disney (which you must be to get the plan), and therefore are planning to eat every meal at Disney, then yes, it can definitely be worth it. But is it THE cheapest way? No. If you don't usually eat much, you can do better. You can share meals, eat only at the counter-service restaurant and eat only the cheapest meals, and do better or about the same. If you plan to eat heavy, sit-down meals, then the plan will almost definitely be worth it.
If you are there mostly because you want to enjoy the park attractions, and you are not going to spend much time or money on the food, skip it, or consider the quick-service dining plan only.
You might want to look here and consider the places you would likely be eating at - this has all the menus and prices: http://allears.net/menu/menus.htm
