One of the main reasons why Mac users still use Boot Camp to run Windows Natively instead using virtualization software like Parallels is its lack of 3D gaming (OpenGL and DirectX) support. In Parallels Desktop version 3.0, now in final candidate development, you can finally use 3D gaming without rebooting.
The other major feature is "SmartSelect," which allows users to open a file in either a OS X or Windows program directly from OS X. You won't have to fire up the Windows program first and open it the long way. New versions cost $79 and upgrade versions are $39 if you buy before June 6.