I can think of a few good reasons, but hey are purely developer-oriented. At my last job, I had an Ubuntu-based desktop because it made virtualization cleaner with a more stable base system—and the company wouldn't buy Macs. Windows ran in a virtual machine, and I developed on Linux, Windows, and a couple of Unixes. For that job, Ubuntu was the best choice I made.

Now? I'm happily running on a Mac.