I have been using Parallels for a few months on my MacBook Pro. Mainly I wanted to be able to run Windows XP so that I could run the Groupwise email client. My district uses Groupwise as our email solution, and the native Mac client that Novell provides is pretty flaky. Otherwise I have to use the web interface to Groupwise which is really lacking in features. Parallels allows me to run not only Windows, but other Intel based operating systems including various flavors of Linux. I currently have Ubuntu also installed. And, if I really wanted to be faithful to my district's technology standards, I could also install Windows 2000. :-)
Anyway, Parallels allows me to run Windows, Linux and Mac OS X all on one machine and at the same time. The new beta upgrade brings a couple of new features including the ability to drag and drop files between environments, and something they call Coherency, which is basically the ability to run individual Windows applications just as I would seperate Mac OS X applications. I can move between them just as you would any other applications on OS X. This is really a great feature and makes the work environment more like I would expect.
In the screenshot below you can see I am running Photostory 3 and Internet Explorer in Windows XP, along with Firefox and iPhoto in OS X. Also note the Windows taskbar situated under my Apple menu bar... The ability to drag and drop files between environments is really great too. The images in Photostory 3 came from iPhoto. One thing they are still working on is 3D hardware acceleration so applications that rely on that will not run, but they indicate that is coming on the next upgrade which will be first quarter of '07.