In my experience, QNX on a desktop has been less than stellar. However, I don't believe that QNX has ever been intended to be a Windows replacement. In my mind, it's used as a development tool for applications that are destined for embedded QNX installs.
Which brings me to another point: While you may see QNX on many small embedded systems, my guess is those particular OS installs don't have the window manager and all the tools that come with the desktop release. OK, no guessing required, all embedded OSes are as stripped down as possible to maximize performance/minimize hardware requirements.
Also, honold has a good point in that RTOSes have specific scheduling requirements that generally preclude desktop usage scenarios but are excellent when predictable response is required.