I think it's a great feature, particularly for new users, but it's too small for people to notice. I wish that there was a text label/phrase next to it so that it would grab people's attention. Our visitors don't have much experience with forums, so cues like this are vital.
I was thinking of using Nodes As Tabs to create a menu of portions of the forum/site that intermediate users might not find. Then I realized that all I had to do was edit the QuickNav template for a better solution. Now all I have to do is draw more attention to it.
Never. Even though the function has a lot of potential, it's too hidden to be useful. I like Waindigo's mega menu and DigitalPoints implementation of the quick navigation menu as both are awesome and useful. While both are very similar to the XF QNM I do not find XF's implementation useful at all.
Never! Although I'm sure it gets used since it was hidden by mistake in a recent version of UI.X and other admins were quickly asking for it's return. I personally didn't miss it and preferred it gone.