It's worth it if you are going to be eating at a lot of participating restaurants with a large party (TIW covers up to 10 persons in one party), and/or making multiple trips. It can also be useful if you prefer to order alcoholic beverages. It's not all that useful if you prefer CS meals as there are only nine CS restaurants that accept it. It's also blacked out on major holidays.
If you will be spending more than $375 on eligible food and beverages in your years membership you WILL realize savings. How much will depend on how much you spend over that. You're also allowed more flexibility in ordering when you have TIW, because it's not the same as prepayment, which is what you are doing on DDP. On regular DDP if you eat at an a la carte restaurant you prepay for an entree and dessert and one non-alcoholic beverage (not including those with souvenir cups or glow cubes). If you order anything else, that's outside cost.