Mono isn't proprietary.
Most things work, but there are a few things that are broken in unavoidable ways. Similarly, the performance isn't terrible, but the optimization is not quite at the same place as the original platform.
Xamarin's rapid work suggests that there will be a 'production-ready' CLR for most platforms (they've got it down for iOS, Android, and recently OSX) soon enough.
Right now, they are stuck in the odd middle, but it is still an amazing value if you are building common code cross platform apps.