I feel for the instructors. Car sickness is common apparently. 
I've been out as a passenger a couple of times on track - most recently in a friends 911 which went very, very quickly around Anglesey .. after half a dozen laps I felt awfully queasy and had to ask him to go back to the pits before I lost my lunch

I have watched a few clips of the experience on You Tube and am surprised at how nervous some people look.
You get a total mix of people on these events and you have to consider that some of them could be the same people you found pottering nervously up the middle lane at 65mph half an hour earlier

On the day I did there was a vast range of speeds and experiences, from the people who pootled around the track in the Lambo to the guy who managed to 360 the Ariel Atom before he'd even passed the first cone

(enthusiasm > experience!)
I did have a couple of laps spoiled when I came flying around the track only to find someone in front of me (no overtaking allowed on my event as it was a tiny coned course), but to be fair the instructors took a pragmatic view and usually told you to do an extra lap or, at one point "Don't bother turning in for the top corner this time, you're much faster than that other guy .. just keep going to the end of the airfield and then turn round up there"
