It takes a while to get used to, but once you do, it reduces eye strain a lot. Especially if you sometimes get too absorbed in work and forget that it's getting darker and it's probably time to turn the light on.

The brightness values are nowhere near extreme enough, and they're not adjustable beyond a few presets. The intent, which it does okay, is making your computer match the color tone of whatever your indoor lighting is, which makes the display more red than it makes it dark. But what you really want it to do is make the display dark enough that it won't keep you artificially awake late at night when the monitor is the only light source. On the Mac, the best tool I've found for that is Shades (http://www.charcoaldesign.co.uk/shades) which unfortunately doesn't have a timer. But binding to alt-space or whatever is pretty much just as good.

As a side note, the effect that monitors and other bright lights have on preventing sleep is the suppression of enzymes in the pineal gland that convert serotonin into melatonin about an hour before bedtime. There's a deletion mutation in the genes that code for one of these enzymes (ASMT) that's very common in autistics and relatives of autistics (which is a non-trivial subset of hackers/coders/geeks), so you might not be benefitting from the darkness anyway. You can solve the problem more directly and reliably by just taking melatonin supplements.

Basically, it automatically adjusts the RGB elements of your display to be more subtle on your eyes depending on what time of day it is and lighting conditions in your room. It does a fair job at it. I highly recommend it.

I've been runnning it for about 3 months, and its great. Every once in a while I'll turn it off to work in Photoshop, and the brightness of the screen is jarring.

It fixed a problem I didn't know I had.