Posting a response to clarify: I answered "never purchase," although that's not entirely true. We DID purchase it in the past, and found that:
(1) even though we booked all the most expensive 1-credit TS restaurants for dinner and went to all the most expensive CS restaurants for lunch, choosing the more expensive entrees at each, we just about "broke even," saving a little over $100 on a party of four in a week (or more like $150 when we traveled with a party of 6 for a week), off of what we'd have spent for the exact same items if we'd paid for them out of pocket;
(2) the amount of food we received was far more than we wanted or needed to eat, and much of it went to waste (including a handful of "snacks" we'd get the last day just to use up the credits, and then give away to others); plus, because it was more than we'd otherwise have ordered, it was clear that if had ordered only what we actually wanted to eat and paid for it out of pocket, we'd have saved a significant amount of money -- for example, left to my own devices, I don't drink fountain beverages or order desserts -- that's a savings of $20/day all by iteself; and
(3) the benefit of having pre-paid did not simplify our vacation any, nor did it make us feel carefree about what we ordered, because there was the constant pressure to "get the best value" out of the plan by ordering the most expensive options on a menu, even if that wasn't what we preferred to eat.
Based on that experience, unless there is a compelling reason to do so (e.g., it's offered for free or at a discount, or in one case we were traveling with a family member and wanted to cover some of their costs without their knowing -- we told a little white lie and said we had free dining even though we didn't), we forgo the dining plan.