Just cloned only the menus from our main site into a sub-site. Mostly good except that the old menus were erased (may be no way around that so that's ok) but one minor bug... the menu buttons were out of order.
A slightly more problematic issue is that although I only had 'menus' selected in the settings, a lot of my widgets got scrambled as well.
We had two sidebar menus which disappeared (when all of the menus were wiped of course). That's understandable. But it the widgets disappeared and even a few totally unrelated text widgets were pushed into different widget areas which is mucho bizarro. (Pretty sure that's the technical term for it.)