For Desktops/Laptops in a business or very large environment, Windows will win every time. Any other OS is unmanageable but I can modify every Windows workstation in a corporation in a few clicks with Group Policies and Active Directory. Novell is hoping to change that with their purchase of SuSe and other technologies, but they've basically screwed their customers repeatedly, abandoning them so it will take some time for people with memories to even consider Novell again.
For servers, it depends on what kind of server. Web server? Sure, run Apache on RedHat Enterprise or any supported flavor you like. Print/File/Security/Etc.... now you've got problems if your environment is very large, back to Windows winning again in most cases. Even mail servers are not a clear cut choice to go with a Linux based solution like Qmail. (Though people love to mention that Hotmail runs on Linux systems running Qmail) For specialized needs, *ix is very valuable and it is reliable BUT ONLY in some circumstances. We are an Oracle shop where I work and must have Unix to support Oracle. It's fast and rock solid, but we ask very little of it. Try plugging your grandma's USB multifunction printer into a Unix system and get it to meet her needs, not going to happen. We're getting closer but we're not there yet.
Mac is awesome, but very overpriced and yes you can get a virus on a mac, yes you can have problems on a mac... and they are increasing as Macs have become more popular. Now virus and malware devs are writing code for Mac. It used to be such a small percentage of the PC population that they didn't bother, but no longer.
Windows XP is the most used, yet the most underappreciated pieces of software out there. When you consider the number of situations it is expected to work in, the number of devices, etc. and the fact that most issues have nothing to do with the OS, but with subpar drivers and software that people install on it, but MS releases patches to fix it... I think Microsoft has done a great job. Your $100 or so dollars for Windows XP that you have been able to use every day for years with free updates is definitely getting your money's worth.
Vista was a disaster. They screwed up by jumping the gun and rushing the OS. It's a lemon like Windows ME was. And definitely not suitable for network environments.
Windows 7 looks like it's going to be a rockstar and just in the nick of time with XP in extended support already and Vista causing problems for so many people.
But for a home user or geek who wants to play around? Use a Windows or Mac for your main workhorse and have a spare PC that you can routinely blow away and reinstall Linux on it so you can say how stable it is.
Of course you can download a Live version of Ubuntu or some other flavor and boot your pc to it, running it in Memory.
But like I said before. ALL of this is changing with cloud computing allowing us to do so much through a browser. You don't even need an office suite installed anymore if you don't want it. So as long as your PC will boot, print, save data and launch a web browser, it will do it for you.
And add in virtualization and things are changing even more. That is one area that Unix is extremely valuable. VMWare, Xenserver, HyperV and all virtualization technologies are a new breed of OS based in linux because it's so lightweight and can use such few resources. Then you run your real servers on top of it.
For a Desktop/Laptop with all things considered including value, Windows 7.
For a Desktop/Laptop if you like overpaying for pretty things, Mac does have some value. I like Macs, but they are 2-4x the cost of a PC so it's hard to justify for me personally.
For a server, it depends on what you're doing with it and how many servers you have to manage.
Operating Systems are like religion to many people and arguing a point, no matter how logical or right you may feel you are, they feel equally justified in their preference.