I'm interested in hearing about this too. We want to go to Universal on our 2nd and 3rd day, so we were going to rent a car at the airport, pick up groceries along the way, and use the car to go to Universal for the next two days and spend the next 6 at WDW only. We were hoping to return the car somewhere more convenient than the airport (at that car center at the TTC or-I think-there is a car rental desk at the Swan or Dolphin), and then take Magical Express back to the airport on day 9.
It does seem like a hassle, so I really want to hear about other people's experiences...