We are an engineering firm with about 30 employees. Before I was hired it was almost 100% microsoft (with four solaris and linux servers). Since then we have migrated the entire engineering staff from windows to linux and nearly all of the tools they use (Cadence and crap like that). The front office staff are staying in windowsworld for now (since they get microsoft docs from clients, that probably isn't gonna change) and some of the servers (mail, file sharing etc) are still waiting to be migrated. The end result? We won't have to pay piles of $$$$ to upgrade everything (Exchange, virus software, etc etc etc) to have the newest mail/web/etc software, and to top it off most people got new machines, which would have cost at least an extra $2,000 for just the windows licenses alone. And don't even get me started on how much it would cost to upgrade the exchange server, we are talking several thousand dollars (for software and hardware, since there is no way the current hardware could run the newest mail server) instead of the ~$500 for a new machine which will replace the current server (which can then get recycled into something else).
Now we can spend our IT money on hardware and quit wasting it on software that quite frankly is inferior. For example, it is funny watching people's faces when I say they can go ahead and open attachments from unknown people in thunderbird now and it won't blast their machine, or that yes they can have a 20" monitor instead of a 17" because the money we saved by not buying the newest version of XP/Office/Virus scanner/etc/etc can now be applied to a bigger monitor instead.