Great, so having previously suffered a rash of sites that permanently took up valuable vertical screen space with fixed headers, they now will all have headers that disappear whenever you scroll down and appear when you scroll up, with slightly buggy behavior on iOS. Has anyone considered what non-technical users will think of this UX idiom?

The effect is exacerbated on mobile devices whose browser chrome follows this very same idiom (for arguably better reasons - at least it's not buggy). So you now have two "competing" user interface effects.