I just wanted to comment on everyone's concern over runaway trim servos. All of the comments seem centered on control switch failure.  I am not aware of a large incidence of this type of failure? (comments welcome) Anyway, I HAVE experienced elevator trim fairlure on a Mac servo.  It was at full nose up, and failed to move, the servo locked up.  This had nothing to do with the switch.  We returned to the field and landed, getting quite a workout holding the pitch.  My two cents, just Keep It Simple.  Everything can and will fail sometime.

       D. Story, ATP