Night Shift Left Me Cold
I've had this on my phone for the last few weeks as I have the beta version. I also use redshift (open source version of f.lux) on my PC and laptop, so this isn't my first foray in to this whole shindig.
Redshift/f.lux on the PC works. My eyes feel less strained, and I generally have a better sleep. Although if I'm working late at night I've noticed I'm more likely to nod off while using the computer with redshift/f.lux on than I do without.
However, I've found night shift to be fairly useless on my phone. It turns on at more or less the right time but there is no gradual change from what I can see. Basically it's off or it's on. No in between (unlike Redshift/f.lux). Plus I think there's only a benefit from this is you stare at the screen for a prolonged period of time. If you're checking your phone or iPad while watching TV with energy saving bulbs it's not going to benefit you.
Suppose it's a mileage may vary, but I think it's fairly useless for the iPhone/iPad.