I tried Mono a few months ago because I was thinking of writing cross-platform (ish) desktop GUI application.
My dev platform is Ubuntu so I downloaded MonoDevelop and load the GtkSharp example (or maybe it was just the skeleton project template). Hit compile, bunch of errors showed up. Apparently I don't have the GTK libraries. Googled a bit, it looks like I had to download gtk-dev, compile, and do a bunch of things before I can run a "Hello, World" GTK sharp app.
I wade through the net and the tutorials and documentations are either for older versions or just weren't there (for example, integrating with NuGet).
It's a bit sad because I'm sure there are tons of engineering effort went into Mono but it just doesn't feel polished enough.
It's open source but lately I felt that it's more of a teaser for Xamarin than anything else.