Personally, I don't think it's a good idea to have the information in your eye-line. It has to be a greater distraction than a unit on the dash, imho.
I also wonder why anyone really needs to look at the sat nav screen when it has clear verbal instructions. Not all sat navs are as good with verbal commands as others, imho.
The technology was given a significant boost in Britain by the Department for Transport (DfT), which has had an apparent change of heart about the legality of using devices such as Google Glass. Last summer it said it would ban drivers from using wearable sat navs. “We are in discussion with the police to ensure that individuals do not use this technology while driving,” it said in July.
However, this weekend the DfT told The Sunday Times that it had held talks with Google about ways of legally using the device. “We have met with Google to discuss the implications of the current law for Google Glass,” it said. “Google are anxious their products do not pose a road safety risk and are currently considering options to allow the technology to be used in accordance with the law.” UK Drivers May Get To Use Google Glass While Driving [driving.co.uk]