Depending on what you have to do, it's not, but I have to edit documents that live in a sharepoint server a couple times a week and I never quite figured out how to do that without a Windows box. That and exchange managing our e-mail makes moving my corporate-issued computer out of Windows somewhat risky career-wise.
Believe-me, I have meditated on this more than once.
You can access basic IMAP from an Exchange server. I was using thunderbird at my last job and it worked fine. Connecting to the LDAP server for the global address book took a bit to figure out. I finally got it all connected and figured out though. If your Exchange usage is more than email and shared addresses I don't know how much further you can get into it without Outlook. Not because it's impossible only because I didn't try to do it.
As far a sharepoint goes I don't have any real experience with it but it looks like you are able to use it on linux but the activex controls will not work(of course):
Sadly, we use Exchange for corporate calendar and contacts. Evolution sort of works, but I have seen it refusing to connect or update stuff more than once.
Had I sufficient memory (machine has 1 GB of RAM), I would go down the VirtualBox/VMWare route.
The machine I have has one 1 GB of memory. I could run Windows under a VM, but that would consume precious resources for the "good side". There is a lot of "heavy" stuff being done in IE (ActiveX for Project and so on).
Yeah, that's a good reason not to, especially if you will need to have your Windows VM running constantly. RAM limitations was why I never tried running Linux as main OS and Windows in a VM on my previous workstation. I had 2GB but that was already being stretched thin by running Eclipse and Visual Studio at the same time.
Believe-me, I have meditated on this more than once.