For a desktop environment I prefer Windows. We have over 14,000 desktops here and there is zero chance that the support calls will stay at the same level or decrease if we through Linux on them. The (l)users will either find or imagine more problems simply by knowing the OS has changed. Also, the (l)users would have to relearn all of their productivity apps as very few of them that are in use here have been ported over to Linux with the same UI.
On the server side, it totally depends on the individual server's role. We use a combination of NT, W2K, and NetWare for standard server ops like file/print sharing, application sharing (Citrix, client-server apps, etc.), and databasing.
On the network infrastructure side of things we use a combination of Cisco, Lucent, Unix, and Linux depending on the specific task at hand. We only use non-unix OSes on the infrastructure side of things for platform specific services like NDS, Active Directory, WINS, etc.
OS wars make me laugh. If I were ignorant enough to go on a "software jihad" I'd have the board kick me out so fast...
Use the best tool for the job when you are in a business environment. Use whatever you want for personal uses. No OS is universally worse than another as they each have their practical and perceived strong points. Chevy vs. Ford anyone?