Years ago when I was reading this (just a couple of chapters, not all of it), it opened my eyes to the power of diagrammatic representation in formal reasoning unlike anything before. I never did anything useful with string diagrams, but it was so fun to see what is possible with this system!
I had a similar revelation when watching 3Blue1Brown's Calculus series. Had they included those kinds of visual representations in school when I was first learning about Calculus, my understanding (and interest) would have been greatly expanded.
Very impressive how some people can create visual representations that enhance understanding.
