This is a good time to mention that Apple, being a bunch of freaking geniuses, actually solved the problem long ago in MacOS. This was demonstrated to me about 6 years ago on MacOS 9, so hopefully the behaviour is retained on MacOS X.
Whether or not the submenu pops up or disappears depends on which direction and how fast you move the mouse. Basically, if it's mostly down and to the right, it sticks on the currently popped-up menu, even if you mouse over another one, because we assume you're trying to get to the submenu (moving to the right, after all!); if it's mostly up or to the left, it pops up the menu you're currently over, instantly, because you're obviously not heading toward the current submenu. And there are never any arbitrary delays.
It's really hard to explain, but it's total genius. The "wait for half a second arbitrarily" technique and the "never delay" technique are massively inferior to this. I have a feeling Apple patented this technique. I wonder if it's expired yet?
Also, when you underline "Gaggle" in MacOS, the gg isn't underlined because it has descenders, but the underline stops a little bit into the gg, so it looks perfect and you can't fake it on other OSes by just asking to underline Ga and le.
That is all.