Depends on how often you go. We tend to buy AP's every other year. We bump our vacations year to year withing a week. Meaning we go on Vacation Sep 12-20, 2011 and buy AP on Sep 12. Next year we go on vacation Sep 4-11, 2012. When you vacation like that you only buy the tickets one time and then don't by until the following year 2013.
The discounts are nice, TIW is great, free parking is good if you are off property, but it's not worth buying ap's just for the discounts. There are too many public discounts that are close that wouldn't make up the difference.