C# the language itself is quite nice, probably one of the best mainstream languages out there. I don't really care tho if it would run on the JVM or native (or something else) instead of the CLR.
Right now tho probably the only popular use of non-ms .net (mono) is Unity (which is btw a pretty big deal in the games industry).
This is the real story here. C#, as a language is fantastic, especially with the most recent versions which have added things like the Task Parallel Library, dynamic keyword, and the new async/await stuff. It's very easy to write a great performing app with C#.
Right now tho probably the only popular use of non-ms .net (mono) is Unity (which is btw a pretty big deal in the games industry).