Agreed. F.lux is amazing. It takes a night of two to get used to, but you do get used to it.

F.lux works by removing blue tones from your screen. That causes the "brown" tones that some posters describe. Your screen will look a bit off (to others, anyway; the tones will look normal to you), but there are a lot of benefits. Blue tones are responsible for a lot of the brightness of your screen, so cutting down on the blue tones results in less glare and eye strain. This change is especially useful at night when brightness hurts the eyes more (though I use f.lux on a low setting to cancel a bit of screen brightness even during the day).

Another great benefit to reducing screen brightness/ blue tones is improved sleep. Blue tones and brightness are indicators to the body and brain that it's daytime (think the sun in the sky) and thus time to be alert and awake. That's a big reason why using electronics before bed has been shown to keep people awake. Remove a good chunk of that blueness, however, and it is much easier to fall asleep if you're using electronics close to bedtime (though I think that games are often stimulating enough that using f.lux with exciting games doesn't necessarily make it much easier to fall asleep).

The skewing of tones can be annoying for pictures, movies, and games, but f.lux can be disabled or set on movie mode to remove some of the blue tones from movies without removing so much that the movies' color looks wrong. As far as games go, I often have to decrease my usual f.lux settings for particularly dark games. It's generally impossible to find all of the necessary inventory items in adventure games set in haunted houses, for instance.