Well I run the evil newbie distro, Redhat.
I started about 2 years ago, but had been playing around with it since about 1997. I wanted to get a cable modem, and decided I would build a linux router project firewall for masquerading. Well, I learned a lot of command line stuff and ended up making a completely silent, small router. I used a solid state disk on chip for storage. Still running 24/7 like a champ.
Then I decided to create a webserver at home using redhat 6.2. I then added mail and domain name services. I now run MySQL database and PHP extensions. This led me to setup a sourceforge project called RIMPS to serve my mp3 through a webpage frontend. I now have my entire 10GB mp3 collection online anywhere in the world that I have a internet connection. It supports searching, playlist generation, seperate login accounts and lots of other nice features.
Next I built another linux server to run Linux terminal server project. This is still an ongoing project, but I have one terminal running and a second one very close.
I switched over one server I administer at work about a year ago. Just switched over the second one, which was win 2000 server until a few days ago. I got sick of patching windows only to be cracked again in a few weeks. It is now supporting our 5 tape DLT changer, which gives us 150GB of compressed backup space.
The main reason I switched was for the challenge. Now I really like the ability to completely control every aspect of security and functionality. It is also a much cheaper hobby than windows. I spend an hour or 2 a week checking out sourceforge or freshmeat for interesting new opensource projects to try out.
My next big step is to switch my desktop machines. They are currently win 2000. I will do that after I complete my Phd dissertation this summer. The nice thing is that I am using open office 1.0 and GIMP, a picture editor like photoshop, on windows right now. This should make the transition to a linux desktop much less painful.
Edit: The newest versions of linux are much less painful to install and use. Granted you must have hardware equivalent to what a win machine would need, but with ease of install seems to come serious bloat. Right now I have an install on a pentium pro 200, which runs quite nicely with the blackbox windowing interface. It's much to slow with KDE or Gnome.