Gordy mentioned KTMs. It's amazing how different a given bike can be made to behave with setup of motor and chassis.
Sharing bikes is problematic, as sting32 points out -- a little different take-up on the clutch, a little different pitch to the handlebars, and you wonder how anybody can ride this thing!?
My son is six inches taller, twenty-five years younger, three classes better at trials. He can ride my bike, but his is completely foreign to me. To each his own.
My wife is about my size. She sets up her mountain bike so oddly I can't even figure out what is wrong with it. She makes it go, I make mine go....