I know that a couple of others have already posted this idea, but I just wanted to add a few points:
If you're staying on Disney property and you plan on spending the majority of the time in WDW, why not grab a bus or cab from the airport to your resort? I know that Mears used to run buses to WDW, but now I hear that WDW has their own. It will certainly be less expensive that renting a car for the duration.
Once at your resort, WDW transportation is fairly convenient and reliable. Aside from the buses, the different modes of transportation could be very interesting as well, especially if you've got children with you. The monorails and boats add to the whole WDW experience.
I can not speak of off-site hotels, as I've never stayed at one, but I believe that many have shuttles that take you to the park gates. Again, I've not experienced any first hand, so I can not vouch for their reliability.
Any time that I've ever gone to WDW, I've only stayed on Disney property. Having never rented a car during any of these trips has always worked out very well, and has saved some much needed cash to spend on the rest of the vacation.
Scott