Already have. Not much noticeable difference, aside from the app switcher and Siri.
The only differences I've noticed have been negatives, honestly. Like weird
stylistic choices and that unnecessary space-waster in the music app telling me I have it set to only show my locally stored music that I can't make go away.
Also no fix for the phantom, seemingly corrupted "Other" data that started taking up half of my phone storage after I updated to 8.4.