Used it a couple of time to get to Ellis Park. Just pitch on time at place where bus departs from (this place normally has ample parking for your car). You normally get an armband when you go on the bus. Bus drops you off at event, and then like the other guys said, make sure about departure times and make sure you are there on time. Bus then takes you back to where you got on.
Edit : most of these services require you to purchase your ticket in advance online.

