I recall a long group ride, I traded bikes with other riders just to feel them out. In one instance, the guy I swapped with could only stand a few hundred yards on my bike, the fit of my bike was just not right for him. In the other case, I couldn't stand more than a few hundred yards on a different guy's bike -- but at least I helped him realize he needs a much bigger bike (or at least higher seatpost and longer stem), and his comfort has since improved dramatically.