I am not yet sold on micro kernels as opposed to a monolithic kernel, but then OS's is not my thing. Microkernel architectures are usually more robust, and generally a more elegant architecture, but there can be a performance hit. One cool thing is that the Mach kernel was born on multiprocessor machines, and that coupled with the G5 chip's "mother" being originally designed for multi-core, multiprocessor setups means the Mac is one hell of an SMP box.
I am going to have to get myself a mac. I've got a PowerPC RS6000 that I haven't used in well over 5 years. OS, is obsolete (Aix 1.25) and Aix is up to 4.xx something or other. Sad to see it sitting unused in my closet, I'll bet with a good OS it would still be a useable computer. That could run my web server if nothing else. I liked Linux years and years ago, but it's pretty much a bloated, OS these days and dog slow on the computers I have, P1s. I once put Linux on a old Sun box (Sparc 1+) and totally ruined it. FreeBSD might be OK, tried a couple of times to get a download to put on one of my old P1s, but never could find a PC install.
I'm happy Mac decided to go Unix for an OS, that puts Macs back on my radar screen and thier $500 box is in my price range too.
I'll get mac one of these days.