Taking Christmas out of the equation, the dining plan is only good if you would've spent the same or more anyway. The only way to figure this out is to look at sample menus, add up the costs, and decide.
In general, if you are going to a lot of character meals and buffets and also drink alcoholic beverages, the plan will probably either pay for itself or come very close. If you eat appetizers, share dishes, don't drink alcohol, and/or have no desire for refillable mugs, then the dining plan isn't for you.
You are going to get every opinion possible and if you look around even a little you'll find maybe 100,000 threads discussing this issue into the ground.
You will get posters who'll say it doesn't matter if it pays for itself, they like having their meals paid for in advance even if they've overpaid for them. On the other end of things, you'll get posters like me, for whom the dining plan (any of them) has never even come within chewing distance of what I spend on food at Disney, which has always been way way less than any of the plans.
I don't know if there's a premium for dining on Christmas Eve/Day. Others will certainly respond and let you know.