Ok, I just took the ear light into a dark room with my jetBook, to see how usable the light is. First, holding the jetBook about two feet from my nose, the ear light illuminates the entire jetBook evenly (not just the screen). That is the plus.
There are two minuses. Since the light is striking the screen almost perpendicular, you do get screen glare. If I angle the screen about five degrees to the left, the glare is gone and the screen is very readable (I have the light on my right ear). The other issue is that I have a hot spot in my peripheral vision, which is annoying. I guess that once you concentrate on what you are reading, you may not notice this any more.
The ear light seems no worse than any of several other designs that I have seen or tried over the years. The even coverage is better than many. The ear light uses a standard 2032 coin cell and is very comfortable.
End of unintentional mini review