As a 20 year Apple/Mac person (I have a 2 G3s and a G4 in the house) and a 10 year Unix person with Amiga and Windows boxes for seasoning I thought I would add my datapoint.
I'm surprised at how people declare OSX to have such great Unix capabilities. I find its command line to be very thin in what's available even with Fink installed.
It is unquestionably Unix but compared to the fabulous riches available in any Linux distribution I don't find it to be at all compelling (I use Debian so I'm particularily spoiled).
It's too bad that Tim O'Reilly and others are "Switch"ing just when KDE 3.0 and Gnome 2.0 are bringing some great desktop applications into focus. Add Gstreamer, Xine, Quake 3 and wine-based CounterStrike into the mix and Linux has a very compelling desktop.
To be blunt... these Unix people are leaving for the land of closed-source mystery meat shareware and commercial applications on proprietary expensive hardware.
The KDE and Gnome people have worked really hard to deliver great opensource desktops. Listening to people casually dismiss all their great work by saying "just not there yet" is pretty sickening. Especially when Linux is my 10 hour a day desktop environment. Oh well.
I'm really pleased that I made the full-time switch to Linux, I find OSX and Windows to be completely uncompelling nowadays. With the wide variety of powerful flexible tools, fast beautiful desktop apps and an OS thats runs on any machine on the planet, I'd be a fool to go back to those worlds.