Quick and semi-serious reply...
OS X is based on BSD, which is cool
XP is based on NT, which is uncool
... and another thing about XP that bugs me. The default interface, at least in Home and Pro, looks to me like it could be marketed as "Fisher Price My First Desktop". I realize that that is nitpicking and that it is easily changeable for those likely to care.
All that being said the machines I use daily at work are W2K and WXP and the system at home that sees the most use on a daily basis by myself and others is WXP (though I often use my FreeBSD box). I've never owned a Mac, but what I have seen of OS X impresses me. I was like the proverbial "kid in a candy store" when I first played around with my friend's Powerbook G4... "sendmail?! Cool! bash prompt?! Cool! cron?! Cool! gcc?! Cool!" You get the picture...
What I really would like to drop into my environment is an XServe...