I did the 8 minute drive (Indy car) at Kentucky Motor Speedway and it was the most exhilerating thing I've ever done.  Check in was a bit slow but after that everything moved along fine.  Main thing to remember (which they try to beat into your head before you drive) is to trust that your car will stick to the track in the turns, even at top speed (I hit 157.21, at least according to the certificate that I got afterwards).  I have no idea how many laps I got in because it was so intense and I was focused on being where my spotter wanted me to be.  It's a lot of money based on a per minute rate but if you enjoy speed, you should do this at least once in your life.