I believe that wearable computing devices will become popular in the future. For right now, I personally don't believe that wearable computing devices is well designed to be comfortable enough to be wearing around all day. Do you remember your first audio headset? I do, and it did not fit well on my head and the audio sound was not the best. But today, I have a wonderful headset that fits well on my head and the audio sound is beyond outstanding. My point is that over time wearable computing devices will improve and if there is a demand for it, you can expect it to do well in sales like mobile devices are doing today.
I believe part of the problem is in the efficiency of biomechanics (not only efficiency of piezoelectrics). The SF Exploratorium had a nice demo showing that very vigorous pedaling of a stationary bike could - sort of - light up a 60W bulb. In other words, maybe 10% efficiency (watts out/bioenergy in). I also know that the MIT Media Lab demonstrated the shoe-based generator many years ago, but people wearing them found them tiresome. And of course the high efficiency materials are expensive. Maybe a lot of incremental advances will overcome these hurdles.
Energy harvesting shoes fail to get traction...that's a headline! Now I just need to go research to see if there's a story behind it or not.
If anyone has any contacts working in the area, let me know. I suspect if there's a problem it could be we don't have the capability of harvesting/using enough energy cost effectively yet...a worthy research topic.
We worked at Virtual Vision in the 90's on a wearable computer. It was a project with Honeywell and Boeing to capture the manual for an airframe on a belt worn computer. The goggles were way klunkier than Google Glass. The solution was clever but was "looking for a problem". I.e. we had lots of clever ideas how folk would use wearable computers but at the end of the day the market decided it was not ready. Some of the same dialog comes up with google glass. I think it's cool and have seen a couple in San Francisco, but think it'll take a "killer application" to really drive it.