I've used AAA three times in three years, so I know a bit about it.
The discount is off the
room portion of the package. The particular percentage off the room varies depending on the resort you choose and the season you go. Note that the 15% or 20% is NOT off the entire price of the package; it's only off what the room costs within that package.
Below is from
www.mousesavers.com.
AAA and CAA Resort Hotel Discounts
American Automobile Association (AAA) and Canadian Automobile Association (CAA) members qualify for "room only" discounts almost year-round, though there are a limited number of rooms available for each date. You can generally save about 10-20% at the Disney resort hotels, depending on the season.
In general, the discounts run as follows:
During Value Season you can get up to 15% off at the Disney Deluxe and Disney Vacation Club resorts, or 20% off at the Value and Moderate resorts and the Ft. Wilderness campsites.
During Regular Season you can usually get up to 10% off at the Disney Deluxe and Disney Vacation Club resorts, or 15% off at the Value and Moderate resorts and the Ft. Wilderness campsites.
During Peak Season and Summer Season you can usually get up to 10% off at the Disney Deluxe and Disney Vacation Club resorts, or 15% off at the Value and Moderate resorts.
The only time AAA/CAA discounts don't usually apply is Holiday Season (around Christmas) at the Value and Moderate resorts.
To answer your question if it's worth it, in my case it is. We save about $400 off our annual trips to the Polynesian with the AAA discount. We go when there are no other discounts available, so AAA is the only way to pay less than rack rate. Wilderness Lodge is a deluxe resort, and you'd be going in Value season, so you can plan on saving 15% off the rack rate of that hotel. At 6 or 7 nights, you'll be saving a few hundred dollars.
Here's a link detailing the pricing of various rooms at the Wilderness.
http://www.mousesavers.com/wildernesslodge2008.html
In addition to the discounts above, if you book a package through AAA, you get a few perks like a discount shopping card for Downtown Disney, preferred parking at the parks, and a storytime with a character in MK.
Enjoy your trip!