I have Mono projects in production. Xamarin Studio is pretty nice these days. I don't have a lot of performance sensitive .Net code so I don't notice any real difference between Microsoft's and Xamarin's implementations.
One problem I'm running into at the moment is I'm trying to port a vendor's reference SOAP Service over to Mono/Linux and their provided secret sauce DLL is blowing up when calling their objects' constructors. I'm yet to pin it on Linux or Mono. Here's hoping the vendor is Mono friendly and willing to debug it. If not we're going to have one oddball Windows server in a sea of Ubuntu EC2 instances.