In the beginning...
Since first installing Linux in about 1995 I have been hooked on Linux. At first, my main reason for using it was because my University's servers were always crowded the night before programming assignments were due. To relieve some of the server load (and actually get my assignments done in the comfort of my home) I loaded up the then-new Slackware 4.0 which included the newly released Linux 2.0.0 kernel.
After slowly getting used to setting up Linux on my machine, I fiddled more and more, installed different free programs, tweaked the OS for my hardware and in doing so fell in love with the beauty of the whole system. I found that more of what I really wanted to do, which was develop software, was so much easier and cheaper than the old way that I was used to. That old copy of Microsoft Quick C and Borland Turbo Pascal which I paid good money for quickly became unused compared my new Linux development environment.
Stardate: Unknown
In the time between my first introduction to Linux and the present, many years have passed. All of my PCs run Linux as their main operating system, and only occasionally have to boot into Windows for a little Dark Age of Camelot. My work PCs are pretty much Linux only, as are all my current work projects. Large corporations are starting to see the benefit of running an alternate OS. Linux seems to be flexible enough that it has many areas where it can make a niche for itself, such as embedded systems, network servers, workstations, and even desktop systems in my opinion.
Update 2005/05/18: I am now hooked on World of Warcraft, which works under the latest CVS version of Wine or the commercial Cedega quite nicely, so I have been seeing much less of Windows lately.
We can rebuild him...
Until about 2004 I strictly installed only Slackware on my PCs, whether upgrading or doing a fresh install. The simplicity and light-weightedness of the system plus my familiarity with it were the biggest factors for me. The other Linux distributions, such as RedHat, SuSE and Mandrake were too flashy for my liking. They tended to be more desktop user oriented rather than power-user or developer oriented and the package managers never agreed with me.
Lately I've been playing with Gentoo a lot. At first I installed it on my laptop which was new at the time and then after screwing up the Slackware install on my desktop, also installed Gentoo there as well. If you are like me and love to build from sources all the time, but hate having to track package dependencies manually, then Gentoo is probably the Linux distribution you are looking for.