I'll qualify the title shortly, but first let me recap. I've given Parallels Desktop a pretty decent chance. Only recently I wrote that I thought it had improved dramatically in the latest build. I've recommended it to friends, and they've purchased and installed it.
Yesterday, I was running Windows XP on a Parallels virtual machine on my quad-core Mac Pro, which has 4GB of RAM. It was smooth enough. I may have had as many as 20 other applications open (I generally just never quit applications—with 8 cores and 4 GB, why would I?), and then I logged my wife in as a second user. Then I launched a different Windows XP virtual machine in another instance of Parallels. As if that wasn't enough, she then launched some monolithic Java application inside that—and that's when it happened. According to Activity Monitor, the CPUs were doing essentially nothing, yet the whole user interface locked up. I couldn't even get Finder's Force Quit dialog up to kill Parallels. Finally, the disks stopped, and the mouse froze, and it was all over bar the power-cycling.
So while the title is true, I did happen to be punishing the machine a little at the time. I imagine it was a memory issue in the end. But, locking up to the point of requiring a power-cycle is a really poor failure mode. I think I'm over Parallels Desktop. I'm going to check out VMWare Fusion.