What are your current travel plans? Where were you staying, for how long and what type of passes were you planning on buying? What are your travel dates and are you booked with a pacakge or room only. If you can post that info, I'd be glad to plug it in to the formula I used in figuring if an AP was the best option for my clients who didn't have one already.
As far as what it would mean if you purchased an AP...you would be able to park hop and it would give you 1 year of admission to the 4 major theme parks. You'd have some dining and retail purchase discounts and other entitlements in addition to the room discounts (which are not guaranteed). You can see all of the 2005 benefits at
http://www.magicalmemoriestravel.com/APBenefits.html A regular AP would not cover admission to the water parks, Disney Quest or Pleasure Island but you would get a discount off of purchasing tickets for those parks. You could get a premium AP that would include those admissions but honestly, you'd have to go quite a bit to make up for the cost increase.
You don't have to have the pass purchased before booking the AP discount so you can make sure that it is going to be the least expensive option AND that there are discounted rooms available for your dates before you actually purchase the pass.
Like I said, if you post your travel plans as they are now, I'd be glad to help you analyze it!