I like having my speed up for funsies so I know when I'm breaking the limit, but that's about it. The page on my head unit I use for structured training rides doesn't even have speed.
Have you used or seen a HUD before? For example if you play video games, how many times might someone die while looking away from the screen? What about compared to glancing at their health, or Ammo? While the uninitiated think a HUD blocks your vision, it actually rests in a position (and is translucent) in such a way that you see through it and any glance would be milliseconds comparer to seconds of looking away. Plus your peripheral vision. Would notice changes to things while looking at the HUD compared to looking completely away.
yes I used Google Glass for a year. it definitely obscures your vision compared to not using it.
it's funny you mention games because most games I've been playing have been moving away from having a HUD visible all of the time, and only show it contextually.
I tend to believe from other people's reports (also in this page) that Google Glass works differently from more recent systems. Translucent augmentational data, augmentational data and environment co-present and just eye-focus away in focus do not seem to obscure the vision. Personal experience, though not used at high speeds.