I have let my two sons drive it, my brother and three friends. One son does not really care about it and really does not care about it. My other son is interested in it, has gone over how to change all the attachments and maintain them. He also understands the limitations but does not always take precautions (example while trying to maneuver to pull a stump the tractor started to tip and instead of backing up to solid ground he stepped on the pedal to make it go as fast as possible and forced the tractor forward). Eventually he will inherit the tractor so I guess you can say I am trying to teach him... He will pay for any major damage though.
Of the friends, one bent the front bucket and let his wife drive it who raised the bucket all the way up full of logs which promptly fell back on top of the hood denting it. Another was leveling his yard and while looking back at the york rake crashed into a tree knocking the FEL loader off level (i.e. one side would hit the ground before the other).
The last friend know how to use it, has repaired (welded) the backhoe frame for me when it cracked (manufacturing error and frame was eventually replaced), added chain hooks, repaired backhoe bucket that I cracked, helped repair damage caused by other friends, etc... As of right now he is the only friend I will ever let borrow it again.
My brother used it and I did not see any evidence of abuse so I guess you could say he could still borrow it, but my wife is saying I should not let him and I should go do the work...
There is a lot to be said for NOT letting people borrow your equipment even if they claim to know how to use it.