Let's work the math.
If you take 1 trip a month for 13 months and on each trip you eat a TS meal. If the bill for 2 comes to $50 each time (low for WDW, but easy number to work with) and you generally tip 18% ($7.50 on $50 bill) then your total food cost for these meals would be $747.50 ($57.50 X 13).
With TIW your breakdown on the same meals would be:
$50 bill - 20% TIW discount ($10), then add automatic 18% gratuity ($7.20) for a total bill of $47.20. Multiplied times 13 trips and you have $613.60. Add the $100 TIW card cost and your final total is $713.60.
A total savings of $33.90.
Plus, you have the added benefit of being able to take friends and/or family and get the same discount for them. Just remember, the person whose name is on the card has to be the person who pays. No split checks.