If you are getting a rental car, it is a total waste of time and money (if you are staying at one of the conference hotels, of course).  The convention parking wasn't free in 2000, and I don't think it has changed.  Plus the parking was all the way in the back, and the buses dropped you off right in front of the entrances closest to the conference.  The buses are quick and easy, and you usually don’t have to wait long to catch a ride.  The only exception to this rule is the Universal Party.  Then, you have to wait in line to go there, and then, depending on when you leave, you may have to wait in line to get back to the hotel.  But, it is a worth it.