QSDP can be a good deal for adults. It's a "no-brainer" good deal for kids 3-9. Having four kids in your family, you'll probably save a lot on QSDP, assuming that you would normally be buying a kid's meal for each child at each meal.
Kids' QSDP cost is $8.99/night, which includes 2 CS meals ($4.49+tax each) and 2 snacks, plus the "resort mug". Ignoring the value of the mug, you're paying less for the QSDP than you would for the 2 CS meals ($9.56, including tax), so you get the 2 snacks for "free" in addition to saving a tiny bit on the CS meals.
Adults are trickier. If you're just going with burger + fries + drink, you can probably get by on about $11 for a CS, after tax. That means you'd have to maximize your snack value to pretty much break even, though you'd get the mug for free. On the other hand, pick the higher-end entrees and include the dessert, and you can easily get to $20+ value out of a CS meal if you pick the right places, such as Wolfgang Puck Express, Cosmic Ray's, Tangerine Cafe, Sunshine Seasons, Flame Tree BBQ, and Pepper Market (but probably at least $13-14 for less "optimal" choices).
So, if you're going to be eating the CS meals anyway, then yes, I believe the QSDP is good value, made better by having more kids on it.