Heard about it, use it. This doesn't make an EInk screen out of your LCD, however. All I can say about it is that it does make looking at an LCD screen more comfortable later in the evening, but still, you don't get the contrast EInk gives you. Neither do you get the battery life, for battery-powered devices.
Also, since this has nothing to do with Calibre, I'm moving it to an appropriate forum.