I have used it, and for what it is I like it rather well. If I was in the business of building real-time systems, I would choose it over Linux in a heartbeat.
At the time I learned C on it Minix was brand new and Linux was a distant future. QNX is a rock-solid RTOS. There was, once, a desktop version with a GUI, but I never used it.
I may be wrong, but I remember it could boot, compile C code and run vi from a 360k floppy.
Has anyone here used it and liked it more than, e.g. Linux?