"Seriously why would anyone use Ubuntu over OS X? This OS is a mess."
Josh just tweeted this a little while ago, and I thought I'd give an answer. I use Ubuntu (and Lubuntu on my laptop) as my daily driver. I really tried to like Snow Leopard. But it felt like poison, as some might say, or at least like a toy (compare Paul's piece from 2011 on The Condescending UI).
There are two reasons I use Ubuntu over other OSes. The first is that I found I work much more efficiently on the platform. I have to use Windows as well for my work, but I always take a hit in productivity when I do. But I'm sure someone who's spent as long with OS X as I have with Ubuntu (7 years now) would feel the exact opposite--and rightly so. That's also probably the reason I haven't strayed from Ubuntu to another Linux distro. Even though I like trying them out, I'm just more faster and more comfortable in Ubuntu.
The second reason is that I like learning. Running Ubuntu is sometimes like owning an old car. It'll break down on you. If you're someone who likes to fix cars, this is no problem. I'm one of those people. When something goes wrong on one of my computers the process of going from problem to research to fixing means that I get to learn new things about Linux and how computers work. The benefits of this process outweigh, in my opinion, the less frequent "problem to frustration to reinstalling the OS."
There are other reasons, too. I feel that I have real control of my computer. To use another car analogy, using Ubuntu feels more like driving stick than an automatic. I also like the it's all free. And it's breathed new life into older machines that otherwise wouldn't have a purpose. But the two reasons above are why I've stuck with it.
All this said, I've never told any friends or colleagues that they should replace their operating system with Ubuntu. Someone once said that Linux is only free if your time is worthless. If you feel that learning about Linux isn't worthwhile for you, then that statement is really true.