I've got the GPE on my One M8 now since Saturday.
Looks nice enough, I like the lockscreen notifications, the improved facial recognition (i.e. I dont have to stare at my ugly mug every time I unlock it), the animations are cute, and even the white has grown on me - its a more colourful and pleasant experience. Much prefer the notification toggles now, although quite why you are still unable to go into the wifi settings by long pressing the toggle is beyond me - the old way was somewhat counter intuitive but the new way is a step backwards completely.
I would say that the phone maybe feels a smidge slower than before but hardly anything to worry about really.
Battery life despite the much lauded 36% improvement seems about the same.
Not a bad upgrade, but not exactly earth shattering, and I suspect we've reached a "peak" of sorts now where we only get small, inconsequential updates going forward.