I'm very happy with it. Like Retina before it, you only really notice it when you don't have it. Very quickly it becomes the 'new normal' and going into settings and switching it off can be a real surprise in many common lighting conditions.
Night Shift is a much bigger effect, and isn't calibrated to your environment, but I like it. I know some people think it's too extreme, and it is noticeable. I'm using it on my 6+ right now and it obvious it's on, but again if I switch it off temporarily the blue-whiteness of the un-shifted display looks horribly sickly. But then the warmer colours work particularly well with the HN colour scheme.
I agree, the effect of turning it off is extremely jarring. It's kind of like using f.lux late at night and then turning it off. The blue light becomes blinding.
Night Shift is a much bigger effect, and isn't calibrated to your environment, but I like it. I know some people think it's too extreme, and it is noticeable. I'm using it on my 6+ right now and it obvious it's on, but again if I switch it off temporarily the blue-whiteness of the un-shifted display looks horribly sickly. But then the warmer colours work particularly well with the HN colour scheme.