Not sure about 20% off dining. We didn't get that, LOL!
What you do get is a discount on the site/room. If there are no other discounts to be had, as is often the case these days, then it's better than nothing. I think it's usually 15% off the campsites.
We did get a discount coupon for Planet Hollywood but didn't use it. You have to book a package for that.
If you already have a room reservation, you're best bet for tickets is through a ticket broker. Check mousesavers.com for info on reliable places. Sign up for the newsletter too!