We did the dining plan a few years back when it was more affordable (and all the kiddos were under 10). We really enjoyed trying out all the spots at Disney, and it was absolutely worth it then. But for the money, we felt overall that the food and service were generally average, although the atmosphere was often exceptional.
Now we take a different approach. We stay offsite and schedule a few special meals to enjoy the atmosphere, but we mostly eat at the rental home or enjoy other options.
Last year, we did Chef Mickey's, the Plaza (Mostly to avoid food lines, although it was nice) and Akershus Lunch with Princesses.
Next year with a group of 28-30, (if possible), we'll likely do Biergarten and Ohana.
DH and I weren't all that impressed with the "Signature Restaurants" as they stack up against fine dining options where we live, and while we are in Disney we look for a unique dining experience versus an upscale one.
For example, lots of great accolades are posted here for Le Cellier, but a I know lots of places near home to get a great steak in a warm atmosphere, so its unlikely we'd eat there.
And Flying Fish has great food and nice presentation (although its loud), but not up to par with our favorite spots at home so we skip it.