Makes it darker, but is essentially useless.

It does make it darker but all it does give a grey tint over everything making the clarity of your screen terrible. And considering it just puts a tint on your screen and cannot change the factory minimum brightness on your computer it doenst give you more battery life time either....its quite usless. You can see this by putting your actuall screen on low and then out your screen in negative (control+alt+cmd+8) and then turn dimmer than dim on and off. You will see what i mean.

James Pyne about Dimmer Than Dim