I vote for click-to-activate rather than rollover-to-activate, for the first time opening the menu.  Then as others have said, once the menu is open, rolling over other items highlights or expands them.
 
We had an app last year where at first we implemented rollover-to-activate.  It was like the one you describe with a menu bar across the top and menus that drop down the page from the top.  It turned out to be annoying behavior that a bunch of users complained about.  We observed people deliberately moving the mouse around the edge of the screen to avoid putting it over the menus, so the menus wouldn't open up at times they were not wanted.
 
- Mitch Gart

 

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