I used to use it and love it, but after a while I got tired of the inaccurate color profile in the evenings and realized I was disabling it more often than not. However, if you're just staring at documentation and code, it does wonders for your eyes. (An easy way to check how nice it is is to disable it late at night and feel your retinas come close to combustion)
The bright flash of daylight color temperature when the MacBook Pro's switch graphics cards is the only downside* that I've experienced using f.lux. After you've been using it for a while it's painful to use a computer without it at night.
