I worked on a virtual reality heads up display project years ago. It got scraped because it was to strenous for people to focus on a heads up display that close. I think the fighter pilots mostly have heads up display on a screen in front of them so they can focus through the display to the outside. It will be interesting to see how many people were reading their GPS coordinates when they hit that tree.
The HUD (Head Up Display) in a military jet is focused at infinity....meaning, your eyes are focus'd out to the horizon (virutally infinity) and there's no extra focussing needed to view the HUD information. You in fact look right through it to see the terrain, kind of like an overlay. Very easy to use, ONCE you get used to it.