I really enjoyed using QNX (I had a job porting Unix programs to QNX---it was not that hard of a job actually). What really blew me away as the inherent transparent networking. I could, from the command line, run a program on my computer, referencing a file on a second computer, pipe the output to a program on a third computer which sends the output to a file on a fourth computer. Really mind blowing stuff.

And fast. I knew the owners of a company that sold X Windows commercially, and their fastest version ran under QNX, using the native QNX message passing facilities. The fact that the QNX kernel on a Pentium was only 8K in size was also mind blowing.