I'll start by seconding everything Oleo says.
I second that.
And then there's "I third that". Trying to pull out the words to say "OPPOSITE RUDDER NO AILERON" without scaring the student even further is an intense personal challenge and you never know when it's going to be needed. In that respect, you can see an obvious improvement in the mental speed of instructors. But there's another side to it as well, you start becoming a mind reader. I can tell by the way someone taxis (lookout, small corrections all the time) how they are going to cope with a C152 dropping a wing when they are in a turn. The situational awareness for the aeroplane, yourself and your student means you figure out reasonably quickly (I'm talking a few months here) what behavioural signs go together.
Taking that into the airline context, if you have been an instructor (even just doing it for the hours) and now you're a F/O, you apparently have a better understanding of the behaviour of your Captain and how to say "You're about to stuff up" thereby preventing an incident before you *need* to remember how to recover from one.
Despite the length of that last sentence, you also learn what's important to say and when to say it quickly.
In short, it's excellent as a personal challenge, a meeting ground for new people, and all in all a life-changing experience. Wouldn't miss it.
Safe flying
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Confident, cocky, lazy, dead.