I'm a longtime Mac user and unix lover, so OSX is basically a fantasy come true for me. The GUI is easy enough for anyone to use, yet the ability to open Terminal and have a unix prompt satisfies the most hardcore command-line junkies out there.

I don't have much more to add over what previous posters have said, except that I love BBEdit for my Perl stuff.

Update: I've put my money where my mouth is, and purchased a MacBook Pro (aka Intel laptop) for myself my wife. ;) One thing that REALLY impressed me was migrating applications and documents from our main iMac to the laptop. I connected a firewire cable between them, put the iMac into "target disk mode" and clicked Continue a few times on the laptop. Within two hours, the laptop was a clone of the desktop, including applications that ran without any extra configuration (which I had fully expected to do), network, logins - EVERYTHING. Truly an amazing piece of software.