We got in on the free dining plan promotion last fall and paid for it on our trip in January. Obviously, the free dining plan promotion from last year did it's job and hooked us in!!!
We really, really liked the DP because it gives you a great opportunity to try out different restaurants that you might not normally be willing to splurge on. Even our "cheaper" TS meals (character meals, buffets) ended up costing about what we paid for the dining plan per day so we ended up ahead with our CS and snacks. Our January trip was for 9 nights and 5 of our meals ended up costing over $100 for DH and I (DD is under 3 so she either shared our meal or we paid OOP for a kids meal)! Oh...and I'm a vegetarian, so my entrees were generally the least expensive thing on the menu. Our cheaper meals were around $75-80 after tax and tip were added in.
I think the only disadvantage is that the DP gives you A LOT of food and if you aren't big eaters or have issues with only eating half your meal, it might not be for you. DH and I LOVE to eat and try new things, so it definitely wasn't a problem for us!!!!