I purchased an AP for the first time in my life this year, and I live in New Hampshire (so we can't just drive over for the day). We are
DVC members and planned 2 trips in the same 12 months specifically to "save money" with the annual pass. As a DVC member, there is a discounted pass available. Our two trips are August 2011 (10 nights) and April 2012 (7 nights). Here's what we saved as a family of 4.
Tickets: $1600 for AP, $2300 if we bought 2-7 day passes
Tables in Wonderland: For $75 dollars as an AP holder, you can purchase a tables in Wonderland card. This gives a 20% table dining discount (while adding an 18% tip) for the full year.
Accommodations: As DVC owners, we don't need accommodations, however, those discounts are generally made public so that would be an additional savings.
Merchandise: There is a 10% (i think) savings on merchandise in many of the shops.
Overall, I'm not sure the savings of the hotel would be enough to justify the cost of the AP, especially for one trip. It eventually seems like most discounts are eventually offered to the public. What I have heard of people doing is buying one AP for the tables in wonderland card, as you can cover 10 diners with one card and with large groups, that can add up.
Hope this helps.