I just "made the switch" a few scant weeks ago. I had been an avid windows user... no... even supporter *hangs head in shame* for quite some time. My reasons for such were: It accomplishes everything I need it to in a reasonably speedy manner. I had used Windows 95, and then went to 2000 (both of those were probably the respective best of their group, IMO), and never really had a problem. In Win2k at school, I had an uptime of 56 days straight, and I use my computer for _everything_.
However, I had been interested in exploring GNU/Linux for quite some time. How can a guy be a self-proclaimed "geek" and not dabble? I just never got around to it. Recently though, MS has just... been up to _soooo_ many retarded things, even more than usual. Patenting the button... c'mon.
So, that was my deciding factor really. For me, GNU/Linux has taken a lot of adjustment in mindset of how the OS works, and has been a learning experience, which ended up being really fun. It's to the point now after about a month where I can accomplish as much in Linux as I could in Windows, even though I can't for the life of me pick a GD Desktop Environment
-Jesse