I was a die hard PC user since 2nd grade - only OS besides DOS I'd ever known. I laughed and called Macs lame and that they were for non-technical people (as I got older anyway), joked about the mitten mouse, had a great time making fun of the stupid smiley interface, etc., but never really knew anyone that had one.
Then my brother got one and I could see, first-hand, just how much better everything was. He was able to do cool ***** with the built-in apps that I could never do with my Windows OS. I was blown away by how the apps were integrated with each other but without bloat and endless, useless options. Only the core of the app config was there, only the stuff that mattered. When I found out OS X had a unix core, my mind flipped over on itself and that was it.
To that point my idea of computing was tweaking windows for performance, getting the latest upgrades, hanging out with the guys from the PC store backroom, all that stuff. After seeing my bro using his mac laptop, I realized a whole new level of usage. Actual usage to communicate with the world in a seamless, integrated way, where everything came together in an organic and natural sum. Sounds cheesy, and maybe it is, but it was truly a revelation.
Soon after I bought some iPods and a mac mini, immediately squeezed it to death, so a couple of weeks later, bought a Dual G5 desktop. After about a year with it, decided I wanted to go mobile, bought a MBP. Now waiting for the new MBPs, of course.
Not once since I got my mini have I ever considered going back to Windows. I did install Vista on an old PC to check it out but it had severe problems so I turned the PC off and moved it back into the closet.
I haven't tried W7, I hear it's nice. But I'm so overly content with OS X, I doubt there's anything in W7 that would cause me to even consider using it.