The first time I tried to read this book I was mathematically immature and put it down after the first 15 pages or so.
When I came back to it after a year or two of mathematical meanderings, something clicked and I "got" category theory. I don't know if this book deserves the credit, maybe it was just time.
I particularly like the emphasis in certain places on quivers. I found it helpful to think of categories as "just" a certain kind of directed graph, one where the edges form an algebra.