We had the TIW card previously but have since decided not to renew. You need to keep in mind that when using the card, while you get a 20% discount, they automatically add an 18% gratuity to the bill.
Also, the TIW card doesn't include most CS places - The only counter service places that accept TIW are:
The 4 value resort food courts
The POFQ food court
Artists Palette at Saratoga Springs
Pizzafari, Restaurantosaurus and Flame Tree at AK
Of the table services, Teppan Edo and Tokyo Dining don't accept it, nor do most of the TS at Downtown Disney. I also believe it isn't accepted at Yak & Yeti (it wasn't when we were there).
Also, TIW has blackout dates on New Year's Day, Easter Sunday, Mother's Day, Independence Day, Thanksgiving Day, Christmas Eve, Christmas Day, and New Years Eve.
For us, it really wasn't worth it as it basically just paid for the tip. Whether or not it's a worthwhile discount for your family depends on your eating habits. You will need to spend more than $375 at eligible restaurants to get any value from the TIW card.