Cheap, cheap, cheap
I'm a dual-platform user with a ton of PC software I have no interest in paying to replace. I run an old Celeron desktop with dual 7200rpm HDDs and good optical drives, a Toshiba Portege 4005 PIII notebook, and, of course, my Mac Mini (1.42/80gb/WiFi/Bluetooth) w/acrylic 23" Cinema Display -- everything connects through a 802.11g router. Periodically I do have to restart my Wintels to get a solid network link through Bonjour, but other than that I have no problems save one:
I don't want to pay big $$$ for an OSX copy of MS Office Pro.
And no, iWork/Appleworks doesn't cut it. And then there's programs like the USGS digital orthophoto quad utility. And the GPS mapping software. And so it goes.
Truth be told, I'll probably hang on to at least one Windows machine for the foreseeable future, although I hate the networking portions of XP Pro. But if I didn't care about being able to do cross-platform stuff, I'd probably go all Mac...eventually! What's stopping you?