I can't remember an epiphanal "a-ha!" moment.
I remember that by 1999 I had migrated from OS/2 to Linux as my primary computing environment, and I've never really left; I ran Linux for a bunch of years, and then slid over to OS X, where I've mostly remained--the host OS on most of the modern stuff I own is either OS X or Linux, with other Unixes usually running under emulation (I have a NeXTStation and three Sparc machines of varying vintage, all running period-appropriate Unixes, when I fire them up, but they don't stay running; the MicroVAX comes frustratingly close to running but doesn't quite, and I haven't even powered up the VAX 11/730 I got a couple months ago to see how close it is to usable). Turning back the clock farther...
In 1998 I remember someone saying, with a mixture of awe and horror, "I've never seen anyone use a GUI as just a place to stash a bunch of terminal windows before." I also remember screwing with dialup scripts to run PPP on my Linux machine at home, which must have been '96 or so? So that early I was clearly living in it at least enough that I didn't want to leave to fire up another OS (although I also remember learning far too much about PPP on OS/2), and spending enough time connected to the Internet that I wanted a network stack running all the time.
I remember Cygwin on NT and ... EMX, I think it was? ... to let me use and build Unixy-feeling things on NT and OS/2 respectively, which suggests that while I was still using them in the mid-to-late 90s I kinda hated them. (That's not quite fair; OS/2 had some nice points.)
In 1992 or 1993 I remember fiddling with definitions inside kernel include files to make my soundcard, my parallel port, and my modem all work at the same time (again on Linux), and not finding that a big deal (I had some nonstandard IRQs set up to get everything to play nice together, IIRC). I guess that was also about when I was hand-editing my partition table to multiboot a 386DX/25 of my very own so it could run Linux and DOS/Win3.11 off the same drive.
I remember some very early Linux experiences (late '91 or early '92) as my first exposure to bash (the Sun workstations at school ran SunOS and my environment, at least, was csh, which was certainly less unpleasant than /bin/sh) and realizing how vastly much more I liked using bash than csh, as well as the difficulty my muscle memory had transitioning from esc-completion to tab-completion. I remember that I didn't really speak C at that point. Though come to think of it I didn't really get _fluent_ at C until the late-ish 90s.
The first Unix system I used was something, probably Xenix, on a Dell '386 in a physics lab at UT Austin in the summer of '89, and while I didn't really "get" Unix at that point, I knew I liked it better than DOS, and it let me access Usenet, which was a huge deal. That was the same summer and same lab where I discovered the flight simulator on the SGI IRIS. I'm sure that hardware was expensive and used to do complex nonlinear dynamic simulations for people, but it was also certainly used for flying pretend planes in what seemed then like an astonishingly realistically rendered 3-D world. That summer was when I made the choice between Emacs and vi, and that choice has stuck with me for 30 years. In two more years the core of my .emacs file (which I inherited, obviously) will be old enough to be president.
Adam