Which is why I'm planning to put quite a few of our services onto Linux servers. Possibly not the proxy yet, but for E-mail I'm trying to talk them into going with Zimbra over Exchange, and my print server will definitely be a Linux system.
Back-end stuff is fine. It will run and run and run and the end users will generally be unaware of it unless you make it known. Proxy, email, file serving, backup, web hosting, dns/dhcp, printing - all ideal services to run on Linux and even BSD.
And a lot of followers are evangelical. I suppose its just my natural rebellion against being preached to that makes me sceptical of some aspects of Linux.
Being free in cost is just one advantage of Linux. Don't forget the free and open nature of Linux. There's no vendor lock-in, there are no CALs, its incredibly stable, secure by default and many better features than the MS efforts.