I bought a TIW card on my last trip, last month...we were three adults (DS and a friend in their 20s and me, a "child" w/a middle (going on senior) aged body) and we ate mostly TS...it would have been great (we all liked to get appetizers and at a couple of meals we had an appetizer to start, and then another for an entree)--clearly superior to DDP for us on that trip.
BUT, I discovered that almost every restaurant we went to offered a 10%-20% AP, Disney Visa or
DVC discount...there wasn't a single restaurant we went to that didn't--and a couple did not take the TIW, but did give me the same 20% discount for my AP (or Disney Visa or DVC)...I quickly got confused and lost track of who was offering what...but did come away with the sense that I would have saved almost as much, or maybe more after factoring in the cost of the TIW, w/out the TIW--equal discounts were pretty much always available.
Now, that may be one of those "it's the rotten economy/promo" situations, and so may not last--but neither will the TIW (mine expires 2/28/11; I bought it 1/10/10)...
Anyway...that was my experience last month. I was going to use it again on my Feb trip, but Snowmageddon cratered that trip...don't know yet when we'll reschedule it...and we're going again for T-giving, but TIW doesn't work on T-giving Day (but will on the days around T-giving)...so I don't know if I'll break even or not...but one good thing it did prove to me: the DDP is NOT the be-all and end-all of cost-efficient dining at WDW...sometimes it's great, but other times it's just not...
Good luck w/your decision...if you have the patience to research which restaurants offer which discounts at which times, you could probably save a mint, maybe even w/out buying the TIW...