You can read about all the current AP discounts here:
http://www.wdwinfo.com/discounts/discounts-ap.htm
The 10% merchandise discount is nice - and it includes food items purchased at Disney merchandise locations. We get lucky with the dining discounts at about 50-75% of the places we like to eat. And it only takes one AP for our entire party of 3-4 to get the 10% discount.
AP Resort discounts are nice but spotty — they are offered late, and only on Disney’s surplus room inventory. But I have used them several times to upgrade from a moderate to a deluxe like AKL at a very reasonable cost.
The only time I’ve ever used the dining plan was during a “free dining” promo, during a trip when I didn’t have an AP. It was OK, but I missed the flexibility of being off-plan. The year I had
DDP, I had SO many snack credits left over! I used them on rice krispy treats for everyone back home. At least it was popular with my relatives!