The main advantage of an Annual Pass, is the discounts off of WDW resorts. As an example, AP rates for Sept 04 for All Stars are $49 per night and Animal Kingdom Lodge at $109 per night.
Other benefits include free parking at the parks, discounts on mini golf, some restaurants and stores. Also, only ONE member of your party needs an AP.
If you arent staying on site and dont plan to return within the year, then to be honest a Fun In The Sun pass maybe better value as you get all main parks AND the water parks, Disney Quest and Pleasure Island (you would need a Premium Annual Pass to cover these) for 30 consecutive days. Possibly one AP and the rest of your party with FITS tickets???
Here are the prices for the Annual Passes:
Annual Pass (Ages 10 and Over) $379.00 (All 4 main parks, MK, AK, MGM Epcot)
Premium Annual Pass (Ages 10 and Over) $499.00 (Premium pass includes 4 main parks, Water parks, DQ and Pleasure Island)
Hope that helps a bit.
Allie