The Mono C# compiler is considered feature complete for C# 1.0 and almost complete for 2.0 (ECMA). The 1.0 compiler has received more testing and is considered more mature than our 2.0 compiler, bug reports can be filed on our bug tracking system.
Mono's C# compiler is called MCS and it is able to compile itself and many more C# programs (there is a test suite included that you can use). It is routinely used to compile Mono, roughly four million lines of C# code and a few other projects.
The compiler is also fairly fast. On a IBM ThinkPad t40 it compiles 18,000 lines of C# code per second.
