As a Linux and Mac OS-X user, supporting a large group of RHEL users and XP users I definitely second this fellow.
If we leave aside the Ad Hominum attacks and AstroTurfing lets look at my experience with the three.
The RHEL systems are rock solid and enterprise manageable but have limited functionality that is very fine, stable, and secure for science, LAMP/Java serving, and custom applications. Very configurable and if you have top notch SA's the same can be done with CENTOS or other Linux so the lock in is not as bad as MS. Still some lock in exists because it costs a lot in house to replace and maintain the RH management pieces we need for compliance reasons. Good security with only our SA team having root and occasionally having to beat users for attempting go around our procedures. (Lightly) The security benefits from fair security in the latest distros, a small number of hardened configurations that we deploy, and of course from not being number one on the blackhat agenda.
Downside - We have tons of management and other users that MUST have Project, Visio, etc. Ek is dead right on need for legacy applications. Companies like VMWare may help virtualize our RHEL systems using unix underneath - but they want to run their management pieces on top of Windows. Even document and spreadsheets become problematic as docx types mean we have to upgrade any desktop unix to the latest Office substitutes and still only get 95% compatibility that gets complained about. So other then SA's unix or linux desktops just don't fly. Of course we also need trained unix SA's who generally don't fill in well on the Windows side and they aren't cheap. I will say they are worth it though and generally leverage their numbers to a much larger base of installed computers per SA.
MacOSX - Very usable, solid and configurable underneath. Cheaper to maintain for hardware and AV. We do our mail AV at the edge so the pass through argument doesn't much apply to adding separate AV costs.
Downside- generally not an option as management acceptance and higher initial cost are issues. Popular with admins ( and myself ) but not nearly as secure as we'd like given pawn2own results. ( Apple does seem to be improving there - realizing the yearly embarrassment kills a major Mac OS X selling point. ) However still a great deal better then the Windows desktops for security. Argue among yourselves about the reasons. ( Some will become obvious below )
Finally Windows. We have XP because we are large and the roll out of 7 is endlessly delayed. Various Windows servers as well for AD and other services. Very user friendly - because we have a helpdesk endless deploying tiered updates by night and blasting out images followed by quick restores from backup after problems. Beautiful support of MS's arcane formats and legacy products that are the lifeblood of an army of pointy haired types who keep the enterprise pumping.
MS support of this house o cards is excellent - they know it has to be. Like IBM back in the day - behind the product is an army of FE's and a mountain of documentation that is constantly changing.
Downside: Even our MSCE's don't really understand under the hood. Don't get me wrong they can tune, deploy, and configure but underneath they need canned rules and configurations and host of third party security products. This isn't a put down, frankly they are constantly in school and are heroic keeping the systems running. Also part of this is the management above not committing resources to get 7 out here. Where we have beta'd it, it's a definite improvement. Our XP desktops though are constantly suffering issues when the latest 0-day sweeps through. Part of that is the 'anything runs' on Windows. Yes indeed - Adobe pdf zero days, Flash zero days, backup software zero days, IE zero days, and god know what other new exploits that we constantly rush the AV fixes out for. This is on top of a 'secure' baseline that is always annoying to users - who we don't allow admin to. We encourage Firefox but many times the pointy haired just can't stretch to that strange territory. I do get that as number one in the OS world for business adoption and home use they get the big target painted on their back.
And cost - trust me not only does Microsoft get their licensing pounds of flesh for OS and apps, but then they make clear you must pay for an endless list of AV, anti spyware, monitoring, and other 'razor blade' services. At least the Unix like OS vendors and community start with a commitment to fix their own security holes rather then have AV and Antispyware as an 'add-on'. Then add license management software to keep track of all those lovely subscriptions leaching away at the IT budget.
If I was at a start up, I'd make everyone from the CEO down to the mailgirl pay a twenty percent penalty on their salary if they used MS on a desktop or server and run the remaining desktops and servers from the various Unix based OS's. I'd have a single pair of Windows systems ( so the current infection can be clean while the other runs ) for legacy support and conversion. Pay a base MSCE salary plus a piecemeal bonus for fast conversion or apps support. Have a second Windows SA to support the first and any desktop users. I know - just a dream - it would never work. You have to admire the MS model of patents and cruft support. "'Dia' can't print this 'network' diagram." "My excel fonts are off."
As the King of Siam would say: "Et cetera, et cetera, et cetera!!!"