I have odften wondered about using a GPS. I am usually very adapt at getting around anywhere I travel....however, when it comes to Disney, even after 9 trips, I have no clue how to get anywhere. DH and I are flying down in Dec and he loves getting a towncar (yep I have tried to get him to go DME, but no luck, yet). It is like I am in a Twilight Zone or Disney Vortex to the 5th Deminsion---Like Groundhog Day---the same way to the parks each time but I just can't seem to remember the landmarks so each trip from the resort to anywhere is like a new experience......then again, I am blonde. 
Any descent unit with current maps should do just fine for you. I have a Magellan and love it, if you get a Magellan they will give you the newest maps on SD card for free, just the cost of shipping the card. New maps for the Magellan are available everywhere i.e. best buy, walmart and so on. So future updates are as easy as grabbing them off the store shelf. Before we left home i entered all of the hotels (Cont, Poly, Pop) and walmart so all i had to do is just pick the places i wanted from the list of saved address. The directions were accurate in all cases and since we go for a min of 10 days we drive around quite a bit.
We were at Disney World in June 2010 and decided not to take our personal GPS to use in a rental car since we have read about the high number of "break and takes" in Orlando. When getting our rental car at Budget they caught me in a weak moment and I took their GPS for $60.00 for the week since he assured me that all of the Disney places were on it. As it turned out the thing was worthless both on the property and off the property. It took me 7 miles out of the way to find Target. A Target that had been open 1 year was not on it and it was within a mile of the propety. The truth is you do not need GPS - just follow the signs.

and our house is 250 years old LOL! I also installed the mousesaver gps files. So handy! I had started trying to locate various WDW places and their addresses and putting them into the GPS. That got very confusing. The mousesaver files was MUCH easier. We do not normally drive around WDW but we do drive to WDW from New England so a GPS is a must.