It's hard to argue with Parallels' might. It's the leading VM on OS X by a long shot, and the features just keep rolling in. Perhaps the most significant new feature is 3D graphics support, which would theoretically let you play your Windows games under virtualisation.
We say theoretically, because tests on our review Mac (a 1.8GHz Core 2 Duo iMac with 1.5GB RAM) showed a distinct lack of 3D performance compared to a Boot Camp XP installation. That said, you'll be able to resurrect games from a few years ago pretty easily.
A new Coherence mode feature called SmartSelect associates Windows files with Windows applications, and vice versa. Double-click a Word doc, for example, and it will automatically open with the Windows version of MS Word.
Parallels Explorer, another new mode, lets you browse your Windows VM as if it was an OS X drive. The lines are being blurred to such an extent that it doesn't feel like you're using a VM; just a slightly sluggish Windows system.