If you spend long days in front of the computer, it's important that the monitor is comfortable to look at. f.lux is designed to help with this by automating screen brightness as the sun goes up and down.
f.lux consists of a really smooth graphical display that shows the current position of the sun. In Settings you can choose a brightness that suits you for the day and the night, and enter your latitude. I didn't know my latitude either, but if you search for your nearest major town or city in Wikipedia you'll find it! If you click the sun on the interface it gives you a quick preview of the dimming effects that will happen over the 24 hour period.
f.lux does its job without problems. It features an override button, so if you're doing color sensitive work you won't be disturbed by a changing screen. It could be improved by automatically finding your latitude, or at least being able to calculate latitude based on your named location, but that's a small issue. How useful this is depends on your computer usage - in an office where the light doesn't change it's a bit of an irrelevance!
While it's not difficult to change screen brightness, f.lux means you never have to think about it again.