As with any dining plan (QSDP included!) the best test to see which is right for you is to take a hard look at what you are expecting and desiring out of the trip.
If you would prefer to dine in the way DxDDP suggests (at least 2TS per night, several signatures, and 3 courses at most meals), then it should work out for you.
In my experience...
2010 - It was great. I never left restaurants too stuffed to move, and I saved ~$200 or so (solo). I was never too concerned about maximizing the plan either, as I had known what I was looking at ahead of time.
2011 - A bit less great. The food and such was still good, but my appetite lessened over the past year. I left many entrees and apps half-eaten, and flat out skipped dessert twice. There wasn't much different from the planning side either between the trips. I also picked up a cold for a few days of the trip, which really shot my desire to eat way down. (I even skipped Citrico's due to this!).
Would I do it again? Probably. If I go with Mom for F&W after she retires, I'd likely spring for it, but I'll have to space the meals out much better than I did this year.
Reasoning? I prefer TS to CS and like to have 2 sit downs per day (most commonly lunch & dinner). I prefer to try several signature restaurants during the trip, and since I'm touring alone, the downtime for the meals isn't a bother (in fact, it's pretty nice). I also can plan it all out pretty far ahead of time to help tweak the schedule and improve potential savings. All of which are my key pieces of advice for others doing the same.
I also have a firm grasp on how it works, which really helps in the planning and dining stages. For instance, I know that skipping dessert at two meals isn't going to wreck my bottom line, so I don't feel I have to order them all. I also typically use the snack credits for drinks (SmartWater works well, at $3.50 a pop) and a couple of the higher priced snacks.