Any time I wanted to use mono it didn't work. It wasn't many times though. A couple of times to try a FPS game and also for some sites based on Silverlight. I'm wondering if it is usable now or what's the common use case for it for which it works fine.
If you want to run Windows .NET applications, in particular GUI applications you may have problems.
But I have been using Mono for developing new software (libraries) for a decade and it works nicely. I often don't need a GUI and if I do, I just do that part on Windows later.
If you play mobile games and have seen a Unity3d splash screen, then you have already used Mono. Silverlight is not Mono. Also Silverlight plugin is being displaced by the HTML5 plugin (shipping with the browser).
we successfully host http://backrecord.com with it and this is now a very substantial project. I find there are often issues to work through when deploying the website (hanging out for ASP.NET vNext), but nothing that can't be overcome. We have background processes that do a lot of mathematics. With these, we find we can generally deploy what we've been testing with the Microsoft CLR to mono/linux and it just works.
