I don't think segways and sinclairs provide the utility to make them mainstream. Not 100% sure Google Glass does either but I think the chance is higher.
I think the segway is a good comparison. It had similar hype ("it will change the way cities are designed") and who uses them today: nerds, and obese mall cops.
Unless Glass solves real problems in a way that is compelling to real people, they will remain nerd toys.
For all the discussion of "cool" and whatever, utility certainly seems key.
We can talk about the Segway's coolness all we want, but it's ultimately not that important. Why did the Segway never become more than a niche product? Because it's pretty much useless! It fills a mostly-unwanted niche between walking and bicycles. Most people can walk. Bicycles are cheap. Given how little it does, I can't fathom how one would have looked at a pre-production Segway and thought, wow, I bet these will sell like hotcakes.
Glass is more of a platform than a product right now. The question of what it's for is still mostly unanswered. Find the killer app for it, and all the questions of creepiness or uncoolness will melt away. Fail to find anything useful to do with it, and the lack of coolness will just become an excuse for the fanboys to use.
But...
> Google Glass only needs to get over its initial days in which it is viewed as awkward and then it will be socially acceptable.
People said that about segway and sinclair C5s and etc etc.
I really hope that Google can manage to make Glass work. Lots of other headmounted displays failed or didn't make it out of very small niches.