I have flown nearly every FS since SubLogic 1.0 for the old Apple. I also hold a (now expired) sailplane license, have sat in the jumpseat of a 737-400, and a C-141, while in flight. On the 737 I enjoyed an hour up front, including the entire landing sequence and taxi to the ramp before I decamped back to my seat.
What I'll say, is FS's have the potential to teach the fundamentals of flight, if the student is paying attention and doesn't turn all the controls to the "easy" side. Unlink your rudder from your ailerons, turn on all the realism settings so your gyro wanders, manually set your IFR radios and instruments, and you have a fair idea of what cockpit management can be like. The dynamics of actual flight will be easy for some, difficult for others. Much depends on your eye-hand coordination, steady stomach, fear of heights, and intuitive attention to detail.
Spending months on an FS will prepare you for some things, especially ground school, air control and navigation issues (if all that's on), and the primary essentials of lift, drag, axis of control, slip/skid, stalls, spins, etc.
Cockpit time has no substitute, either for effective teaching, or the fun factor.