The aim of this project was to provide a machine independent GUI for use on UNIX Workstations. Both IBM and Digital sponsored the initial effort and by 1987 more than a dozen vendors had joined up. It is free software that is distributed under rules much like those of the GNU group. The question is not whether PM is good or not. I think PM is good and sufficient for most people, particularly home users.
X supplement each other.
Under Unix, the GUI is de-facto the X Window System. What the XFree86 port offers, is true connectivity to Unix systems on the GUI base. Commercial competitors, like IBM and Hummingbird claim to offer this as well with their product, but I have yet to see any application developed this way. PMX and EXceed have their market, where it comes to make cheap PCs to a simple replacement for X terminals. Most files in X11 still remain untouched, and compile out of the box.