I have had a distaste for windows for a while, and with OS X including UNIX, i could finally justify spending the money on a new apple machine. Windows has gotten so bloated and full of useless crap, that i wanted a system that could run (and develop software) without having to install 5 - 7 gigs of software. The free developer tools with OS X also were a nice draw, as visual studio .net is like $200 to students, and $700+ after college.
For me was when OS X came out, because it allowed me to completely get away from Windows. Before OS X there were just certain things that the Mac could not do and the PC could. I was a very very very happy man once OS X came out and instead of having to work on two more more computers, including *nix systems.. I could do everything on one computer.. it was a very exhilerating moment!