I've always been reluctant to use Fly Out menus in the sites I've developed. As you can see from the image, these are navigation boxes which display more choices when you roll your mouse over the link.

The mouse is over the navigation link Leasing and displays further choices

Originally my hesitation was driven by the fact that keeping these submenus updated can be a maintenance nightmare for your friendly webmaster:
- they are often ignored by automatic link checkers
- they are written in JavaScript which makes them harder to “Find and Replace” using a code editor
- sometimes they do not work across browsers

I came across an article on the UIE site which for me puts the nail in the coffin for Fly Out menus:

users succeeded more often when they didn't encounter these design elements than when they did

The reason is that studies have shown that users decide where they will click before they move the mouse.

Unfortunately the information in fly outs, rollovers, or dropdowns can't help users decide where to click because the information isn't available to users when they are making their decision. It isn't until after they've decided where to click that they see what the element has to say.

When this happens, users often stop in their tracks, confused, to re-evaluate the information presented to them. And confusion is not a feeling you want to elicit in your users.