I think it's really efficient, and something that I would use when teaching CS. It's tough at first, because it can make us vulnerable and you have to be accountable to another human. Once you get past the initial hiccups, though, what a cool way to work.
It's not about mentoring, but I always come away from a pair programming session having learned a ton of new stuff. Interacting with others who are awesome at what they do is helpful.
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