Our application was originally built for .NET (on Windows). It was successfully ported to Mono, but we continue to have strange problems (machine load average going to 50+ and becoming unresponsive, etc.) which we believe are related to the Mono GC. After spending a good amount of effort trying to fix the Mono version, we decided to bite the bullet and migrate to another platform (Node.js), which has proven to be significantly more stable under load.
I'm a huge fan of Miguel and the rest of the Xamarin crew, and I'd love to see a universal implementation of the CLR take dominance. I think maybe 5 years ago the CLR had a real chance to win in the battle of multi-language runtimes, but at this point I think the war is going to the JVM. (Chalk that up to yet another short-sighted decision by Microsoft.)