It's a cool concept, but I see it as a fad, at least until a less intrusive form factor comes about. I would not be willing to swap my sunglasses for Google Glass. And I had Lasik a decade ago for a reason. I'm not going back to wearing glasses.
Cars generally last longer than phones. Will they have a removable SIM so we can choose a different carrier? Can the cellular radio be replaced when LTE is superceded by MUF (I know it's Monday morning but think about it)?