That menu is a large amount of JavaScript which cannot be guaranteed
to work on all systems due to different browser versions. Also, quite
a large number do not have JavaScript or have it turned off because it
can infringe on security.
In fact, with Javascript disabled on Opera 7.20, the menu is completely
missing; there's no navigation at all on the page.
Note that Google won't be able to index any of the links off the page,
because they're not real links.
If you want to achieve the (admittedly attractive) effect that's present
with Javascript enabled, you really need to mark up the menu so that in the
absence of Javascript it appears as a vertical list of links, with each
main menu item's sub-links underneath it (probably indented). Then use
Javascript to hide the sub-link sections (by manipulating their style
attributes) and pop them up (using absolute positioning, again by
manipulating style attributes) in response to mouse movements.
If you do that, you'll have the best of all worlds: a menu that looks and
works the way you want it when Javascript is enabled, that looks good and
still works if Javascript is disabled, and that enables search engines to
index your whole site.