It might be placebo, but I've noticed a pretty dramatic drowsiness set in immediately following the f.lux "sunset." A lot of the time I will turn it off so I can stay up and get things done. If that's what one experiences at 5pm, I'd call that unusable.

It was absolutely unusable. Here in the north you would get two hours of sunlight during the winter, and f.lux would work accordingly.

Similarly in the summer, you would get only a few hours of darkness, making f.lux almost completely useless except for a few months of the year.

Not dramatic - having your screen turn orange at 6pm is unusable. I'm going to work until 11 or 12pm. Let met set it so I start winding down an hour or two before I go to sleep. It does decrease the contrast of text, making it harder to read.