It's the heart of .NET
It's also the perfect language for Java programmers that want to start into .NET
It's really easy to write applications for mobile devices, web, databases, ...
The best part of .NET & with difference is the IDE.
I can debug an application & change the code on the fly, move the execution flow, check the content of the objects, change them, ... connect to databases & use them to fill the controls, ...
The tools for the programmers is the area where Micro$oft shines the most.
You also can write Direct3D applications easily, and you can use the XNA to develop for your 360 too.
The performance of C# is very good too. It isn't C++ of course, but, really fast too. Also, it's easy to multi-thread an application with C#. It even has components to create background workers.
Personally, i love C# & .NET. The best thing that Micro$oft has created. They make my developer life more easy