I was lucky to have tried glass a few months back and it is pretty fucking cool. Granted I was on an early prototype and they are still a few months out of consumers hands, there is no way the unit I tried is fitting under a helmet. Also, the interface is a bit clumsy, the battery is pretty small so it had to have pretty aggressive power saving, it shuts off in 15 seconds of no use. To activate it again, you have to look up quickly to turn it on and then say "OK Google" to bring up the menu. You frequently use hand gestures on the right side control pad to function it and the interface is pretty awkward. Definitely not motorcycle friendly even if it fit.
The HUD is not in front of your eyes, it is above your right eye. If you are looking straight then you don't see anything. Looking up all the time is not gonna help your riding. That said, the navigation is pretty cool.
I would like to see a better interface, better camera, longer battery life, thinner units and an adjustable HUD, but overall its pretty sweet and I will be getting one. I'm sure there have been several improvements since I last tried it and the units that ship in a few months should behave much better, I just don't think they will be motorcycle friendly anytime soon.