Of course it depends on the length of your stays also. But assuming you are staying more that a few days each trip it will be worth it. We LOVED it!
We did this for our last 2 trips Feb 03 and Jan 04. Some of the really cool things about it were:
1) Like cobbler said the second trip felt "free"
2) You don't feel like you are wasting a day if you use your ticket to go for an hour or two. (Assuming again you are taking longer trips) You may end up at sea world or some other off property thing and end up getting back to WDW at 6 and you could go spend 3 hours at EPCOT and not be "wasting" any days. This worked out GREAT for both of our trips for arrival night!!! We got to WDW at about 6pm on Saturday nights both trips, if we had regular tickets we would have found something to do, but because we had APs we went to the MK. That first year we rode the monorail, had a nice stroll down main st., saw spectromagic, rode 3 rides, saw wishes, and had hot dogs at Caseys while the crowd filed out. It was only a couple of hours but it is one of my favorite memories of all time at WDW. Anyway, it made our entire trip seem longer because the next day "day 1" (especially by that night) it felt like we had been there for 2 full days already.
3) And I love this one... You're trips grow. We planned on going for 5 days both years. But the ticket are "free" for extra days, transportation cost stays the same and the hotel had a 40% discount(with the AP) We ended up doing 6 days that first year and 7 the second. I got to thinking I can add a day for under $50 (hotel and all) So to go from a 5 day trip to a 7 day trip (for a family of 4) was about $100! Thinking about it, it's kind of like getting 4 admissions to the parks and a hotel night for the price of a 1 person 1 day ticket.
We did use a couple of the AP meal discounts and we got some free lanyards and we used the 10% off at World of Disney to save more than I'd like to admit, but my top 3 reasons up there are why I'd love to do it again some day. (2006 and 2007 hopefully!)