I don't use it at all. I sit in front of my computer monitor a minimum of 12 hours/day and don't have any problems with eye strain or sleeping.

I wonder if the people that like f.lux are also the same people that claim that light themes are blinding, because I'm of the opinion that if you find a completely white screen is blinding, then either your brightness is too high or you don't have enough ambient light in the room.