On our last trip, we needed a rental for 3 days to go to Sea World and a weekend beach trip. The other days we stayed on-site. I checked the prices of the on-site rental places, but found that I was able to get a car rental through priceline for the entire trip for less money. So check prices before you rent on-site. If you are going to need the car for more than one day, it might be worth it just to get a rental that you pick up from the airport for the whole trip.
It was also nice having the freedom to not have to use the buses to get from our resort to the parks. Driving there and hopping on a shuttle to the gate was much faster. Otherwise our resort bus had to stop at Fort Wilderness, and at the Grand Floridian before going to the parks, and sometimes the wait for the bus took forever. So after the first couple of days of that nonsense, we just drove to the parks ourselves.