I have been using OSX for about 7 months now, not primarily as a replacement
for MacOS, but as a replacement for LinuxPPC as well.

Mac OSX is a hybrid *nix, a cross between NeXT/OpenStep (which was in turn
derived from BSD 4.1) and FreeBSD.  The filesystem layout is derived from
the changes that happened for NeXT, making directories easier to setup for
netboot and their derivative of NIS, NetInfo.  The display system is derived
from the NeXTStep Display Postscript model, updated to use PDF instead as a
more recent standard.  There are some things which that NeXT heritage brings
to the table that Apple hasn't fully used yet, like built-in kernel support
for clustering, the above-mentioned netboot, and cross-platform binary
compatability (under openstep, the same BINARY could run on Sparc, i386, or
NeXT68k).  FreeBSD brings to the table a better TCP stack, better filesystem
compatability, the BSD's most recent stable port of GCC, and a better set of
tools all around.

I saw somebody posted about Fink, which is definity cool, but since we're
all geeks here, I'll confide in you all that the way to go is the Darwin
ports collection, a direct port of the BSD ports tree system.  It is much
more flexible, and gives you access to a LOT more software.  You can get all
the details at http://gnu-darwin.sourceforge.net.  The guy who maintains it
was just profiled by Apple, you can find that on their site....

I hope this sheds some light on the subject...

Kris Browne
Taylor Corporation Imprinting Group
SGI/Unix System Administrator