We previously used the dining plan, but with two pre-teen "Disney adults," we switched to TIW last March. As others have mentioned, our servers always ask if we are on "a dining plan" at the beginning of each meal. Initially, I thought they meant THE dining plan, and said "no." But then, I realized that when the check arrives, the CM would always have to run back and recalculate with TIW. So I started mentioning TIW when they asked about dining plan. I have not seen any difference in service as a result.
In any event, we LOVE TIW! It's one of the main reasons I'll try to schedule our trips back-to-back in a 12 month period - so that we can get that card!