Apple overcharges for their hardware AND software, and they aren't really great, but they're good enough if you're a casual user. Not saying that people who are "Advanced" Users, or programmers, Hackers, or even Developers use Mac OS/X, but I'm saying that most of the people I know who use Mac only use it because it's not complicated. Microsoft is doing a good job with their computers, Windows XP was fantastic, but Vista was a fail. Windows 7 is a very good and stable OS, but the biggest problem with Windows operating systems are their security. Viruses that destroy the entire Operating System and require you to reinstall your OS. Who knows what else Viruses will be able to do in a few years, possibly even overwrite or delete an entire Partition or corrupt an entire HDD. Linux is safe and stable, and fast, and if "Open Source" means anything to developers, that's a key factor too. I don't like the GNOME Desktop much, but I love KDE, and I plan on using KDE 4.6 soon. Linux can be either easy to use or complex depending on the user and the Distro he uses. Also, not to mention it's free and majority of the tools are too.
*I am NOT a Linux Fanboy, I haven't even heard of Linux a year ago and I started using it a few months ago*