At the risk of feeding the troll, here's why I use Linux:
1) It's cheap.
2) It does everything I want an OS to do.
3) It has tons of great tools and programs available - most of which are free.
4) I like it.
1)I didn't say it was free though because there is some cost associated with buying the CD's, downloading it, or paying subscriptions for updates if you choose to. But it's cheap, and once it's setup it's very trouble-free.
2) Windows does most of what i want an OS to do, too, but I'm a geek, so being able to get inside and customise the OS appeals.
3) I know some people hate the GPL, but some cool software has come out of it...
4) I like it because it's customisible, flexible, and doesn't put arbitrary limits on what you can do. I like the non-manufactured community that sprung up around it. I like that it's close enough to other Unixes that porting software is easish. I like that it tries to keep to open industry standards, rather than creating it's own arbitrary ones. I like that it answers to no single authority, and benefits everyone (ok, Linus oversees the kernel but not the whole extravaganza).
To be fair, I also think Win2K is nice, but licensing kills that fast, and not including an NFS client by default is dumb. MS should have opened up the NT/2K HAL as well, so that the Alpha, PPC, etc thing could have lived on.
The "home" versions of MS OS suck IMHO. In an effort to be "user-friendly", they're too limiting. Eg 1, A friend of mine wanted to use some Chinese on her WinME box... In the end we reinstalled with Win2K, because WinME wasn't multi-lingual enough. Eg 2, Or WinXP's home version networking... Bad bad bad. Eg 3, MS Java. 'Nuf said.
<i>I used to have a girl, but then I got my CS degree...</i>