Well, it still exists, so it's not dead yet 8-)
However, most of the inhabitants have "passed on" in
some sense of the word.
I used A/UX until the last computer I had that supported it
quit working some 7 or 8 years ago. Now Mac OS X does most
of my desktop stuff, and FreeBSD takes server roles.
There aren't many practical reasons for using A/UX at this
point, but if you have questions I'd be glad to try answering