So apparently staring at a screen with a 6500K colour temperature isn't good for melatonin production. A lack of melatonin has been linked to a lack of being able to sleep.
F.lux fixes that by varying the colour temperature through out the day from from 6500K to 3400K, so your screen mimics the light level indoors, from sunlight during the day to halogen or florescent at night and all you need to do to get it to work is tell it what type of lighting you have and your latitude and longitude and you're done.
I've been using it a couple of days now. It does take some getting used to, especially as it makes everything warmer / more red at night, but it does make things a lot easier on the eyes. It's not ideal for designery types, although it does have a "1 hour colour-sensitive" mode that allows you to switch it off temporarily, but it's pretty good for programmers types which includes me.
There's a bit on the Wikipedia article about blue light affecting melatonin and how it regulates the sleep-wake-cycle.
Personally, it's a bit early for me to tell I think. I've not noticed any change in my ability (or lack there of) to get to sleep at night, but I have noticed it being easier to focus on my screens in the evening.
Like Truffy said, it's alreet for anything that doesn't require accurate colour, so editing photos etc. is out the window. Personally I'd rather just try to be off the computer an hour or so before bed.
bastard by name