I believe if you plan on being there more than 8 nights, the AP pays for itself in park use alone. Therefore, any room discounts are an added bonus. I wouldn't necessarily buy an AP unless I knew I was going to be there around 8 nights since the room discounts are not always available.
Here is how an AP worked for me in 2003.
Purchased an AP in May 2003 during my 5 night trip (had a postcard rate so I did not use the AP for a room discount). Used the pass for 5 days.
Had 2 business trips to FLA so I was able to use it on 2 more occassions. No room's at AP rates available on my dates.
Had a 4 night trip in November 2003. The AP rate I paid at AKL was $140 less than the rate I was given when I called Disney for the same room. Over the course of the 4 nights, the savings was $560.
So, in this case it really worked out for me. I entered the parks on 11 days which with an AP is less expensive than if I had purchased park hopper passes. Then I saved over $500 on my room. (But realistically, I wouldn't have paid the non-AP rate that I was quoted so I really probably didn't "save" $500 - but I still feel like I got a "deal").