I don't know about desktop or mobile, but my experience with Mono for web applications has been pretty awful in comparison to Microsoft's implementation. I have

I don't know about desktop or mobile, but my experience with Mono for web applications has been pretty awful in comparison to Microsoft's implementation. I have a tremendous amount of respect for the Mono team, but it's just not a good enough replacement. It's actually caused me to stop writing C#, because I can't stand managing Windows systems, but there isn't really a good alternative.