After reading Justin Tadlock's excellent writeup of Wordpress 3.0 navigation menus (nav-menus), I am seriously disappointed and annoyed at this new feature.
A nightmare to maintain, spaghetti bowl of desynchronization, is what these new menus are. They statically replicate what we already had dynamic widgets for. In database lingo, they violate third normal form.
Post and page descriptions are computed from the page content AT ADD TIME; if you update a post or page, its description is not updated, so you have to remember to manually edit each menu entry each time you edit something. Adding pages, or changing a page’s Order, or changing a page’s Parent, does not change the menu, you have to manually go replicate all your changes in two places.
Nor can I find any way to write a widget that creates menu items, like my http://wordpress.org/extend/plugins/hierarchical-pages/ , which would solve the above problems by creating menus on the fly, based on your current page location, the N latest posts, or other inputs.
In short, the new menus should have been a widget, that could be inserted anywhere, instead of a whole new structure. What a mess.