There are lots of variables as to whether it is worth it.
If you travel with a group, you can save money. If you drink alcohol with your meal or the lounges, you can save money. If you don't have an AP, it can save a huge amount of money. If you have an AP, the calculations get more complicated as you have to figure that discount into your real savings. It also depends on the type of restaurants that you eat at. If you eat at many Signature restaurants, you will save money.
There are many estimates as to what you have to spend to save money with the card. If you don't have an AP, then above about $750 in restaurant charges you will save money. If you have an AP (factoring in that "free discount"), the number is probably closer to $1500. We also go to WDW twice a year. Last year, by timing the purchase of the card right, we got 3 trips out of it.
One other thing, purchase it when you get to WDW. The is activated as soon as it is purchased, compared to an AP which activates the first time you use it.
You also must be a Florida resident, DVC member or AP holder.