It started in my senior year as a CS student - in an operating systems course, we were introduced to a lot of linux stuff and the professor taught Vim as part of his course. At first I rebelled. I chose to develop most of my projects in eclipse/Java at that point and had developed an aversion to the command line. That, plus Vim's learning curve made me hate it at first.
Fast forward a year at my first job at an embedded systems shop writing in pure C, all the vets used vim and I saw how fast they were with it and it made me want to learn. I think my first "aha!" moment was when I accidentally entered visual mode and prepended several lines at once with a comment. After that I was hooked, and while I'm typically one of the only ones using pure vim on any team I'm on, inevitably after a year or so at the job people see how I use it and start asking about it.