One last note about installation: Make sure you turn off the screensaver in your guests. They use the CPU and don't do you any good.

Coherence and Unity

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Both Parallels and Fusion have optional modes for fully integrating Windows applications into the OS X experience. Called Coherence in Parallels and Unity in Fusion, the integration hides the Windows desktop and displays Windows application windows side-by-side with those of OS X applications. In short, Windows apps weave into the OS X desktop as if they were native Mac applications.

Users who like the OS X experience and who plan to run multiple Windows applications will prefer Coherence or Fusion to the old way, which drops the entire Windows desktop into a single OS X window. In any case, switching between the two modes is easy.  

Both Coherence and Unity work well with oddly shaped windows, and they display drop shadows on windows to mimic the OS X look and feel as closely as possible. Both Parallels and Fusion support OS X's Exposé feature, so the VM windows will appear in Exposé tile and thumbnail displays, and you can dock Windows applications for launching and minimization.

Parallels' Coherence hides the Windows desktop, but keeps the Windows task bar for a familiar interface to the Start menu. Fusion's Unity gets rid of the Windows task bar and substitutes its own "launcher" window for the Start menu. I prefer the Parallels approach, finding it more natural. One thing to note: Coherence and Fusion don't work with Linux or any other operating system but Windows.

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Parallels offers a feature called SmartSelect that lets you assign file types from either operating system to the application (Windows or Mac) you want to use. For example, you can set Word documents to be always opened by Office 2007 in Windows so that double-clicking a Word file, even in Finder, launches Word in Windows. Similarly, you could set it up so that clicking a hyperlink anywhere, even in a Windows application, opened the Web site in Safari on the Mac.

Both Parallels and Fusion mount the host file system in Windows. Parallels also makes the Windows file system available as a disk in Finder and provides an offline file system browser for getting files from a Windows disk image when the guest OS isn't fired up.

Regardless of which package you use, one thing the virtual environment can't do is make Windows programs look like Mac programs. One example: Windowing operations in OS X are controlled with the standard OS X traffic light buttons, while those in Windows use the red X box.

The big undo command
Snapshots allow you to save a particular state of the VM, and then revert to it later. Think of it as an OS-level undo command. Snapshots are useful for backing out of an update gone badly, or for testing an application before committing to it. I use snapshots regularly when using virtual machines and have started to wish this feature were available on my standard OS. Snapshots are one of the most useful features of virtualization.