A Disclaimer...I, like many on these boards, am suffering from Dineyitis and my opinions are clouded by a sense of happiness and joy which overcomes me when I think about going to Disney....That said:
I love the "Disney Experience" - Just as the Magic Kingdom was designed as a living movie experience....so too, I view the entire experience once you enter through the gates of WDW as an experience designed to help you feel the koy and "magic" that is WDW. I have stayed off site many times and there are many wonderful hotels which would rival some of the best hotels in the country...none of them are Disney, though.
There is an energy and theme which is constant throughout WDW. The CMs are, for the most part, unrivaled. The fact that you can enjoy The Disney characters anytime of day or night make it special and different. There is nothing like entering into the "city" that is WDW and losing yourself in that experience for a week or more, never to come out, until they tell you its checkout time....having to get into a car and drive offsite, IMHO, dilutes that feeling (although the DD hotels, and Dolphin and Swan are a close second to the Disney resorts - although not themed in such creative and fun ways).
Where else can you spedn multiple days "resort-hopping" - just to see the themes and differences between the various resorts. Such an activity has nothing to do with going to the MK, Epcot, MGM or AK. Its an enjoyable part of a WDW stay which, in my experience is unique to WDW.
One valid point above was regarding Vistana, an excellent timeshare resort (so I have heard). I have solved this dilemna, while still enjoying all the benefits of being "on-site." I bought at DVC

. Those who want a luxury "apartment" experience (even those that have not purchased DVC) can get it by either: (1) renting DVC points; or (2) making a reservation (at one of the DVC resorts - OKW, BWV, BCV, VWL) through Disney.
We'll be in a 2BR luxury suite at Old Key West Resort in October...we can't wait!
See...I told you I have Disneyitis
