I was strangely motivated to port some old MonoDevelop code to GtkAction and GtkUIManager today. Something still seems awkward about that API. I'm not sure it is much of an improvement, if any, from the old way of doing menus and toolbars and it seems to make some things slightly harder.
My main beefs are the callback-based adding of the widgets seem to force me to playing with PackStart/PackEnd tricks and that adding non-button ToolItems seems to take an extra step.
I think the second might be able to be "fixed" in a similar way as [Glade.Widget].