Aeronut
We stayed at the Eldorado (story in itself...) and the shuttle bus went from right outside the door. Getting up to the races was fine (buses didn't run on the first day though) but getting back again was a problem. We weren't the only ones left stranded by missing the last bus back into town and left looking for a taxi.
The last bus went within minutes of the end of the last race so if you want to stay on and soak up the atmosphere or they are running even a few minutes late then you're scuppered.
Price it all up - it'll be a close run thing between car hire and shuttle bus/taxi. I think if I did it again I'd take the flexibility of the car hire option.