Depending on what you are doing, Mono has come very far. The new GC is in and a lot of optimizations have been made. Ubuntu 14 will ship with 3.2.8 I believe which has all the goodies. Apparently the performance is still poor for web development but it works well for daemons/desktop apps. You're probably better starting off with a tech that is cross platform out of the box instead of relying on Mono but if you have an existing C# project then Mono is awesome.
I really like C#. But it's currently pretty close to useless anywhere other than Windows - Mono doesn't even come close.
Hopefully this will close the gap.