It's kinda funny how much can change in a couple months. For me, personally, neither the QSDP nor the
DDP hold any value (well, DDP would for a particular use case). DxDDP is still a value for me (even with next year's prices and this year's menu prices, though it's much closer) so that still works. But now, convenience is winning out over the cost savings.
Since I enjoy dining in the manner of the plans, I don't feel restricted (or forced into ordering stuff I don't want). So, a smaller savings amount (<10-15%) isn't a big deal to me (I know, and understand, that it is for some). Now, I am bad at saving money. It just sifts through my hands, as such, unless I were losing out big time, the prepaid nature makes sense (again, coupled with the already desiring to dine as the plan is structured.) Of course, where before it was a mixture of both savings and convenience, it's now shifted more toward the convenience factor. There will be a point as it shifts further and further away from saving any money though that the loss will outweigh the convenience, and then I'll be done. (Don't worry, I'll still help with the math, only now Havoc and I would totally agree!

)
We have free dining and upgraded to deluxe (together with a 40% room discount as we have booked via disney.uk), so for us it's a no brainer.
But I've just done the maths with the deluxe at full price and only 14 credits used (as we want to do a few quick service meals). For some of the restaurants I didn't even calculate a dessert as nothing appealed to me. Even without the snack credits and the mug, I've already saved money (about USD 20) although I didn't pick the most expensive items most of the time.
We do a lot of character meals and signature restaurants, maybe that's the key here.
Not even taking the double-dipping discount into account, it makes sense that it would do so. Don't know your length of stay, but doing a single credit meal and a signature is a good use of those credits. It helps to keep the food intake to normal levels (so you don't feel all yucky ALL the time) and keeps the value in a decent range. Now, breakfasts (of any type) for adults are typically less value than lunches, but mixing and matching to do what's best for your trip, while maintaining an acceptable level of value received is the way to go!
Now, for 2013, the math stays the same, but the outcome changes greatly. As Havoc has pointed out (here, or in other threads, I don't know, I just stalk him) that you can get away with spending less than $45 (the nightly cost to upgrade DDP to DxDDP) on that one extra TS meal. Doesn't mean that it's a bad value, just makes it more difficult to make it a good one. In other words, it's far FAR easier to make $33 save you money on that extra TS than $45, even though they aren't really that much different.