In my experience, the biggest stumbling block for a Microsoft-heavy business environment is Outlook. If you can get that working well, you're in good shape.
I've run Crossover on Ubuntu, and it mostly works - but there were some odd graphical glitches in Outlook that required some changes in behavior to work around. Not something I'd want to put an average user through.
My best experience has definitely been with virtual machines in seamless (Virtualbox) or Unity (VMWare) modes.