With Virtualbox stable now and under the control of Sun (and still a free download), I really don't see much reason to recommend either Parallels or VMWare over Virtualbox. VMWare/Parallels I guess do support using a Boot Camp partition which is nice, but Virtualbox offers support for snapshots (great for backing up) and disk image compression. Plus, its open disk image format means you can take your VM images to other Linux/Unix platforms. Virtualbox supports something similar to Unity on VMWare, and runs quite well on the machines I've tested it on. I haven't really benchmarked it on OS X yet, but it definitely runs faster than VMWare Player/Workstation for me under Linux. It is also fairly user friendly, providing a GUI to setup virtual machine options.
For those that simply need to run a simple Windows app and/or a game, Wine (which recently hit version 1.0) is a great option. Unlike any virtualization environment (aside from VMWare's Direct3D thing, which offers limited compatibility), 3D games run just fine. Wine/Windows apps run in a window within OS X, and do not require a version of Windows to be installed. Wine is a little tricky to install, and upgrading to the latest version of Apple's XQuartz (available on
http://www.macosforge.org) helps. However, again, I haven't tested a wide variety of apps under OS X yet, but my wife plays Civ IV under Wine on Linux all the time, and it runs like a champ.
Have any of you guys came to the same conclusions that it is really worth giving these two options a good go before planning to shell out for VMWare/Parallels?