I'm a UNIX guy too, and before the Mac my platform of choice was Red Hat. However after a while I
*) Got sick of tinkering with my computer, and building them in the first place
*) Got tired of the constant upgrades required for this security and that new version, oh, and if you want to use the latest chat client you need to upgrade you're whole Gnome environment to something that worked completely different, and not the way you like it.
*) Dinking around with X11. It was fine back when it was the only game in town (heck, I worked with X10 back in the beginning) but eleventy billion different video cards/monitors/GPU got old.
*) Started to get sick of all the different distros, and people telling me to 'use SuSE, no use Ubuntu, no use Debian if you're a real UNIX guy.'
*) Was sick of the Hades of RPM dependencies
*) Tired of enduring he purgatory of apt-get. Yeah, its supposedly easy, but I'd try to use something I'd expect to be included with the install, only to be told "package XYZZY is a part of the PLUGH package, to get this package use 'sudo apt-get PLUGH' in a terminal window" then wait for apt-get to do it's thing.
*) Fed up using Codeweavers Crossover Office to keep up with running necessary Windows apps on my choice of platform at the time. Yeah, it worked, but never 100%.
*) Noted that Mr. Jobs offered an entry level "get 'em to switch on the cheap" Mac in the form of the Mac Mini, so I got one and decided to embark on the Great Mac Experiment of 2005
The experiment was a resounding success. Look under my avatar and you'll see what I'm now posting this on. My wife has an identical one with one less gig of RAM and is black.
I still use Linux and Windows, but it is now firmly in the "work" category. If I'm using a Linux box, whether the one at my desk running Ubuntu, or one of our servers I'm on the clock and getting paid for it.
Oh, and yes, I knew Mac OS X is really a UNIX (and a real one at that, not a clone with all the Gnu stuff thrown in) quite awhile before the switch. Macs were just too expensive at the time for me to consider it then.