We found that buying the annual pass takes a lot of the pressure off of us to "get our money's worth". If the weather is bad or the kids are tired, we can go to a park for just an hour without feeling guilty about "wasting" a day pass for each of us. It's great on arrival day when you just MUST have a taste of the magic, but can't stay very long.
It's also a terrific excuse to plan extra trips to WDW. We used ours for six weeks last year. Once you've paid for the passes, the other trips are pretty cheap. We stay offsite in timeshares, so that is pretty economical. We drive down and don't spend a lot eating out. Hard to find a cheaper vacation.
Sheila