I've waxed and waned about using f.lux (actually http://jonls.dk/redshift/ because f.lux+linux=painful.) I think my biggest problem with it is my office's fluorescent lighting isn't within my control to turn-off/change, so the stark contrast between a screen-gone-red and the screaming white light causes my screen to feel silly.

If I had the luxury of working exclusively in a officespace without fluorescent lighting, I think I could be 100% on f.lux