I moved from Debian to OS X (Tiger) in 2006 because I wanted to try Mac and didn't have the time to tinker with a Linux desktop to make it work. Being tired of not being able to upgrade my programs anymore (e.g., being stuck with Firefox 3.6) and unwilling to buy an OS X upgrade I installed Ubuntu on my Macbook in 2011.
I still use Ubuntu and can't imagine going back. But that's probably because of the tools I use: I think installing and using, e.g., open source programming tools and LaTeX is more streamlined in Linux than Mac.
No. http://images.apple.com/legal/sla/docs/macosx107.pdf: "The grants set forth in this License do not permit you to, and you agree not to, install, use or run the Apple Software on any non-Apple-branded computer, or to enable others to do so."
In 10.4, on the other hand, it's not explicitly stated, only implied: "This License allows you to install and use one copy of the Apple Software on a single Apple-labeled computer at a time."
Where there's a will, there's a relative.