From my point of veiw, I use Ubuntu (generally Linux) for those reasons:

Packages support

• The system is shipped with all applications, ready to use 2 minutes after installation


• Everyone can easily install applications via software center or command line


• If you need, you can choose the version of application you need. Additionally, you can install not-released-yet version from custom PPAs.

Opensource & Community

• Whole system (and most of the applications) are free of charge


• The system is reviewed by many people so many bugs are found and fixed


• If you are in troubles, there are many ways to ask and receive fast answers from community (for example this site)

Security & Safety

• Most of the viruses are designed for Microsoft OS so they won't affect your system


• Thanks to clean system design it's difficult to create really dangerous virus for Linux


• There is easy user management and setting up user rights so you are sure your mother won't break the computer by launching the wrong app.

Programming platform

• Ubuntu (and Linux generally) has outstanding support for programming tools


• You can use almost all IDEs on Ubuntu, simple text editors or terminal editors (vi, emacs)


• There is support for all programming languages


• It's easy to run and test your programs via console


• It's easy to install, configure and use all kinds of servers (apache, database servers, web servers)

Terminal

• Besides the programming itself, you can use terminal to connect to remote servers (ssh, scp, ...) without installing additional software


• Use the power of shell commands and regular expressions when working with your files, processing data or maintaining the system


• If you need to fix/configure something, it's often only about copying several lines from the guide or mail-from-guru and you get the issue fixed.


• You can extend the system functionality with your own scripts launched whenever you need.

Environment

• You have the same environment on your servers and on your PC: You can use tools you are used to use when managing the server. You can redirect X server and launch window applications remotely. You can have the exactly same versions of programs on both server and your PC.


• Somebody says Ubuntu GUI is much easier and comfortable than Windows.

Possibility of choose

• In almost all cases, you can choose from several variants of the application. Unlike Mac OS, you aren't forced to use the "best" app and only it.

And after all, it's Linux. You got to love it :)