The question of "worth" is always a difficult one as it's certainly not automatically going to save you money (anymore).
The below assumes PAID dining plans, if you're getting them for free, the math is a little different!
Basically, when deciding whether or not a dining plan is a good idea for your trip, you have to take into account how you're planning to dine. If you're planning to dine in the structure the plan holds (1 QS, 1 TS, 1 snack per night; entree, drink, dessert at each meal; each person getting their own meals without sharing), then it may save you money. If you tend to order from the higher end of the menus and at the higher end of the restaurant list (cost-wise), then chances are it'll be savings.
For further detail, you'd then want to do the math (I know, I know, it sounds scary, but it's not that bad) to make sure that your plans will utilize the plan in an appropriate way. Go through the menus and figure out roughly what you're family would likely order if you had no plan at all. Go through them again (really, you can do this at the same time

) and figure what you'd prefer to order if on the dining plan. Chances are, that dining plan number is going to be higher

.
Now, apply the savings from the dining plan to the second list and see what you come out with. Unless your "oop" plan was identical to the dining plan...um...plan, chances are the dining plan one will be higher! This is normal, as most people feel like they need to skimp a bit when paying OOP. Take a look at the difference and decide if it's worth it to add that much to your food bill for the increases you are getting by being on the plan (e.g. more desserts, more TS, etc). As a note, if your "savings applied" price is lower than your OOP price, you may skip this step (since, well, it's lower already and all of that doesn't apply

).
I did develop a massive spreadsheet that can be utilized to help you with this if you have Excel 2007/2010, that can be found at
the Cafeen's Disney Dining Plan Cost Analysis Spreadsheet - Reborn thread, it's tedious though!
Now, my big caveat this year is that convenience can have a monetary value, just like flexibility can have. If the values are close one way or the other, you'll have to decide if that difference is worth the convenience of having everything prepaid and budgeted out in advance is worth the difference (if you're losing a little), likewise figuring out if having to stick to the predefined structure of the plan is worth that difference (if you're saving a little). This amount is very personal and will be different with each traveling party! (For me, the budgeting aspect is pretty important, so unless I was losing more than $50-$100 overall, I'd be prone to pick up the plan! For others, I know that unless they were saving more than 10% or so, they'd be more prone to pay OOP).
Now, for free dining, as I mentioned WAY up above, it's a bit different. You'd have to throw in the difference between rack rate for the rooms and the dining plan cost in here. Basically, take the nightly discount, divide it by how ever many people are in your room, and add that as your dining plan nightly cost. So, for example, if I were staying at a $150 per night room and had a 20% discount available, my DDP "cost" would be ~$30 per night on free dining for the entire room. Solo, (since that's how I roll) would still be $30, but for larger groups this number would change. Note that this can go both ways too, if I were at a $500 per night room with a 40% discount, this would mean that DDP would be costing my room ~$200 per night, so, as you can see, if I were solo in this case, I'd certainly be losing out big time!
As far as perks, you save ~$15 per person on a mug (whether you use that mug or not, is a different story

). That's about it. The prepaid nature can be a perk or a detriment, depending on your outlook.