Google Glass was a terrible product. No, I don't say that just because I'm an Apple guy, I say that because it was without a doubt, a terrible product. But still, I had a pair of them, not by choice, but because we wanted to see how they might be used at the office. The short answer was, they were a poor tool for every use we could come up with.
That's not to say that a head-mounted computing device is a bad idea, indeed we came up with many ways such a device could be of use, but this particular device was not capable of doing any of them.
Soon after I purchased the device, I had the opportunity to attend Google I/O in 2013, the year after the device was announced. What was interesting to me was that as I looked around, maybe 20% of the attendees had the devices. That's 1,000 out of 5,000 people, just one year after the initial version was announced. That sounds amazing, even for the conference for the company making the device, but if you looked carefully, you'd realize that it wasn't.
Yes, there were a lot of people wearing Glass, but during the entire two days I was there, I only saw one person actually use the device, and I was watching to see who was using it and how. Everyone just looked silly with a small computing device strapped to their heads; they were not getting utility from it, other than being seen by their peers as having one.
Its easy to look back and see that a product will be a failure; its more difficult to tell that in the moment. This was one of the times where I was able to tell it at the time, and it wasn't that difficult to do.