Whole volumes can be written on whether the plans are "worth it." It really depends on how you usually eat, and how you would ideally eat. For some people, it saves money. For others, it doesn't. For some, they like the "pre-payment" convenience, even if it doesn't save them money. For others, they dislike the structure and restraints, even if it does save a few dollars.
It is always possible to eat well, for less money than the cost of the dining plan.
But the very very basics ---
As of 2012 -- All dining plans come with a refillable mug, which can only be used to soft drink refills at your resort QS location. Some people find these mugs very valuable, other people find them totally worthless.
So if you completely put aside the value of the mugs:
QSDP -- Very rarely a significant money saver. While in theory, if you hunt down the most expensive meals on property (the most expensive 4 meals, out of 1,000), then you could save $10 in a day. But more realistically, even getting above-average priced meals, best case scenario -- you may save $2-$3 per day. Skip any desserts, or get breakfast instead of lunch, or order cheaper meals, and the savings disappear almost immediately. It is very very easy to lose $5-$10 per day using the QSDP.
DDP -- A slightly better value, but still requires eating in a set way. If you use all your credits for lunch and dinner, if you do a lot or character meals and/or get above-average priced entrees, and if you never skip dessert -- Then you can easily save $5-$10 per person, per day. But if you substitute a breakfast for a lunch/dinner... If you skip some of your desserts, if you get a sandwich at the Plaza instead of steak at Kona Cafe -- the savings can quickly evaporate, and you lose money. You also will only save money if you use all your TS credits at ++ priced restaurants, meaning you really need to make ADRs pretty far in advance.
The DxDP-- The most expensive plan. It is very easy to eat "deluxe style" for less than the cost of the plan --- But if you want 2+ TS meals per day, if you want lots of signature restaurants, if you want lots of expensive entrees, if you want lots of desserts and appetizers -- This plan *potentially* can offer the biggest savings... $20+ per day, if you really use up all the credits for TS meals, and don't waste any of your entitlement.