We have stayed both on-site, and off-site many times.
The perks at the on-site hotels aren't all so great for us. The EMH have very heavy crowds, the magical express will not be free after the 50th celebration, and the buses can take quite a while to get you where you want to go, can be VERY crowded, and durring peak times you might have to wait for 3 or 4 before you can even get on one.
Sometimes the 'magic' doesn't really seem to be there at the value resorts. Don't get me wrong, I do love the theming, and services; but after many times being there, you just kind of get used to it, and a plain old hotel isn't much less.
We stay on-site when we either get a good AP rate, or a an actractive special incentive is being offered like the free dining. That was incredible!
When we have stayed off-site, I do extensive research and planning (like many of you DISers), and find hotels that are very close, with minimal traffic, good parking, and nearby restraunts.
Without having to wait for buses, and knowing the Disney proporty so well, we can get to the theme parks very quickly, with free parking (AP), and have the comfort of a rental car. The same thing goes for the afternoon nap/swim. If you know where you're going and are close by, at least where we have stayed in the past, it takes no more if not less than the Disney transportation system. The only exception is the Magic Kingdom. If you're parking at the TTC it does take forever to get to your car, and the buses are right at the entrance to the park.
There are many really fine restraunts near Disney. Having the freedom to travel by car to restraunts, fast food, groceries, ect... is very nice.
All said we are now DVC members, and that is a whole different story. We now have the best of both worlds. We will still get a rental car, but use the buses when we want to. There will still be the times where we may stay off-site when we can't book our entire stay at DVC (not enough points, conserving points on weekends, or other reasons), or stay at another Disney hotel.
Value, in our eyes, is not so much how much money is spent, but the freedom, conviencience, and enjoyment we have. Staying off-site with good planing, has usually been the best value for us, and we have never felt like the 'magic' was ever lost.
p.s. It really makes the day go well when driving the car to and from the parks to have our favorite Disney music on the cd player. Quite the opposite on a packed bus.