I've done this both in Vegas and LA. The 458 is by far the best experience, but the new 911 also exceeded expectations too. The Lambo Superlegerra and R8 were a bore and disappointing. The sound out of the 458 is life changing at the top of the RPM band.
The best way to go about this is to get your package ahead of time, head out to the track and negotiate with them while you are there. Everything is negotiable at the track and they usually have one or two cars on 'special'.
All in all it's a great experience for a few hours.
"I believe that banking institutions are more dangerous to our liberties than standing armies. If the American people ever allow private banks to control the issue of their currency, first by inflation, then by deflation, the banks and corporations that will grow up around the banks will deprive the people of all property until their children wake-up homeless on the continent their fathers conquered."
-Thomas Jefferson 1802