they say it's as "real as it gets"....
but i woldn't rely on it being all that close to the real thing, FS is a great tool for learners of flight because it gives you the opportunity of being familiarized with the flight controls and instruments before ever setting foot in a real plane

the drawback is:
FS doesn't really simulate flight, it uses a bunch of mathematical mambo-jumbo to calculate variables such as lift, drag, speed, wind, etc...
but it doents simulate the flow of air trough the surfaces of the aircraft, so in a way, it's not really flying

FS also does not calculate micro-weather variations such as turbulence caused by trees and terrain, or bubbles of accending hot air, FS turbulence seems to be random thrashing of the aircraft in areas where it's set to happen
i'm not sure how the FS cessna compares to it's real-life counterpart, but it must be at least close enough to give some notion of how it behaves but not to be taken as 100% accurate
FS is not all perfect, but it gives you a good advantage when you get your hands on the real stuff (at least in thoery)

c ya
Moach