I've been using single-menu with focus-follows-mouse on KDE for years and it works quite well. The secret, of course, is that in KDE you can configure the delay for the focus-following. I set it to a 400ms delay so I can easily move the mouse from a window up to the menubar, crossing over other windows without focusing them. It is one of the things I enjoy most about using Linux and KDE.
With all the short, widescreen laptops these days, I would *really* hate to give up my single menubar. Having one menubar for each app wastes *so* much screen space! And really, with toolbars and keyboard shortcuts and right-click menus, I don't even access menubars that often. For most apps I think they should be relegated to a lesser position that uses less space.