I started using Linux because I'm studying Physics, and there are a good many open libraries that allow efficient scientific programming. Then, once I started to really understand Linux, I was flabbergasted by the fact that I can so completely customize Linux to be exactly what I need. Now, I have a desktop, laptop, and server all running different Linux distributions.
I also tried FreeBSD for my server, but I ended up switching back to Debian for it, as Debian is quite easy to set up in a server environment, and even easier to update. I use Gentoo for my desktop box, as it allows me to compile all of my software optimized for my architecture. I use Slackware on my laptop, as it's the best, simplest distro, in my opinion.
I still have Windows on my desktop box, as I have some games that just don't run under Linux. However, I do just about everything else under Linux.