Well, while I agree that you can just follow the signs and you'll get places just fine, I do think it's easier to use a GPS (obviously I'm a bit biased

). There aren't any roads in WDW that are cast-only that are "through" roads, to the best of my knowledge - they all are roads to access a backstage area. A GPS should direct you down a cast road only if it thinks the destination is nearest to one of those roads, and the problem is that the POI for some WDW locations built into most GPSes have terrible locations marked, which in some cases are closest to a backstage road.
So at WDW, if you have good POI that are correctly placed on public roads, the GPS should only route you down public roads. In practice, that's what I have found, at least with Garmin and TomTom products. As part of the work I did to put together the GPS file on MouseSavers, I had two GPSes route me between a bunch of far-flung destinations with a lot of roads in between, like the Grand Floridian and the Downtown Disney Hilton, or Epcot and the Fort Wilderness Campground. In every case I was only directed down public roads, though not always the roads that Disney directs you down. And in a couple of cases where I was routing from a Disney location to an off-property location like a grocery store or a drugstore, it routed me down side roads I didn't know were there, which were off the beaten path but save significant time over driving down the main roads. For example, there's a nice shortcut going from Animal Kingdom Lodge to the Walgreens and grocery stores on Irlo Bronson Hwy by going down Sherberth, and that's a public road. If you follow Disney's directions, you'll go all the way out the south gate on World Dr. There's a nice shortcut going from Old Key West, Port Orleans or Saratoga Springs to Ft Wilderness by taking Bonnet Creek to Vista, while the Disney directions will route you onto World Dr and through the MK parking structure. Again, Bonnet Creek and Vista are public roads. There are lots of places in WDW where the Disney directions to DHS will take you to the south entrance when the north one is much closer. The Disney directions are designed for traffic efficiency, not necessarily to get you where you want to go fastest.
So bottom line I think using a GPS can be a fine way to get around the World, especially if you're going from one hotel to another for dinner and things like that.
Don