I bought several earlier and ended up keeping one. I have it attached to a flashlight by my bedside.
Some people swear by them for their keychains - finding them if they're dropped or in a purse. Other uses I've seen mentioned are for light pullcords, like in closets or the garage; someone else kept them attached to the deadbolt keys they keep on a hook near their doors, in case they have to leave their house in pitch dark/a fire, etc.
Someone said he keeps several in a clear plastic container in his closet, and that that provides enough light to see with.
I find the gadget/neat attraction to be high, but decided I wouldn't be using them on a keychain (they feel slightly fatter in real life than they looked in the pictures I've seen of them).
I don't really notice them glowing in normal room lighting situations - when you get a twilight level of light, they're subtly noticeable, and in a dark room, they stand out quite well - but I wouldn't say you could could comfortably read with it. Perhaps you could pick out a couple words at a time if you held it near a book's page, etc. Wouldn't want to use it to try to find my way in the dark.
Also, green is the brightest color, with the other colors coming in dimmer.
Dave