22 years ago, I traveled to WDW with a friend and stayed off-site. I remember how tired we were driving back to the hotel at night, and vowed never to stay off-site again. Of course, back then we did not have the interenet to help plan vacations and probably could have picked a better choice with a little more effort. I was only 19 then and didn't know too much about trip planning!
It was quite a few years before I got back to WDW, but now travel to WDW about once a year with my own family. We have always stayed on-site, and I couldn't imagine it any other way. To us, the hotel is as much a part of the vacation as the parks; it is not simply a place to sleep (again, that is just for us). I love the theming, convenience, and overall feeling you get from staying at Disney. I like being able to hit the parks in the morning for a few hours, go back to the hotel to swim/relax, then head off again in the evening. I don't think we'll ever stay off-site.