There are benchmarks that shows Parallel Desktops perform much better than VMware. Also there are free softwares like Qemu and Virtualbox, also Darwine. Just listing the alternative, I don't dislike Vmware.
But I do personally use VMWare, since the images are more portable and I use VMWare products not just on OSX. Also, Parallel 4.0's initial release debacle (extreme slowdown) turned me off. Now both of them seem like comparably good, mature and featureful products. Can't go wrong with either, really.