It depends on which parking lot you are parked in, and when you leave. If you start walking to your car about 15-20 minutes before the main headliner is finished, you can get a head start on the mob. Leaving by 11pm means basically no wait at all to leave. If the shuttle service is close to your hotel, and you don't mind paying the $$ it's a good idea.
And, yes, it took 3 hours to get out of the lot after Rage ended. 3 hours of breathing in dust. And, the gas station you pass by on the way out was so busy that all the cold beverages were warm as they sold all the cold ones already.