What's nice about it?

FreeBSD is a nice BSD platform. Darwin is kind of a weird mess, with Unix awkwardly mashed up with Mach and extra C++ frameworks (IOKit etc.) on top.

I'm still not sure of the history there. I remember one of the selling points of the early OS X was that it was a "Certified UNIX" and I remember that no one I tried to sell one too cared.

Was it just the cost of maintaining the certification? The fact that no other OS went down that route?

You can still build it, I believe, they just don't provide ready to go .iso files. I think.