It's a BSD unix with better, more stable accelerated desktop than any unix or linux ever delivered.
Based on hardware that can run windows with just a reboot, no other windows desktop can do the reverse without a lot of hacking and hassle.
Doesn't run out of power as quickly as regular windows boxes do. Let me explain better. I type this on 2008 Mac Pro. Two physical processors. Four cores each. DDR2 memory. ATI 4870 graphics. Nothing extra ordinary. Nothing than I7 couldn't beat to oblivion in any benchmark. Except. It's on my desk for 5 years. The OS runs as fast as it did the first day. It's as stable as it was the first day. It's still runs all the games available to my OS on Steam as it did five years ago. It's as good for video work, fun, browsing and leisure as it was five years ago.
That HAS - hardware acquisition syndrome, that you get on windows boxes, where you feel the need to change graphics every 6 months, CPU every 9 and motherboard every 12? Completely gone. Vanished. I'm cured. During last five years, I added more drives and swapped primary hdd to SSD. That's about it.