TIW is a dining discount card available for AP holders and FLA residents to purchase. The cost is $75 for an AP holder. The card gets you a 20% discount on most WDW table service locations for food and drinks (including alcohol). An auto 18% gratuity also gets added to your bill when you use the card. Only 1 person at the table needs the TIW card (for up to 10 dining guests). The TIW card remains valid for 12 months (some get nearly 13 as it may not expire until the end of the next month). The TIW card does have blackout dates on several holidays. more info:
http://allears.net/din/dde.htm
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DDP is a pre-paid dining plan and is not designed to overall save guests money. (it's a trip add-on that makes the mouse $$$) If you normally order a dinner entree, soda, and desert at the more expensive diner places (like LeCellier) and you get ADR's for those more desireable places, the DDP can save some money.
Personally, I found the 2008+ DDP changes to stack the DDP too much in the mouse's favor, so I started using the TIW card.
You can use both DDP and TIW on the same meal. (DDP for entree and desert, TIW for appetizer and alcohol) That may work well if you get the desireable ADR's and order bottles of wine...
Really you would have to calculate how much you will spend on:
dining cash with any
DVC/AP discounts
VS cash - TiW discount (TIW and DVC/AP are not "stackable", you only get one discount)
VS DDP cost + gratuity and any extra alcohol/appetizers
Some say the TIW "break-even" is about $350 total trip on food/drinks, but that does not necessiraly take DVC/AP discounts into effect..