A few quick impressions of the 640:
The onscreen buttons, just seem "off", like they're not quite right, doesn't fit well or something.
I already miss the screen on the 1020. 640's colors just "pop" too much, liked the more muted colors of the 1020, viewing angle is also understandably much worse. Also miss my glance background.
No dedicated camera button is a bummer.
So far it beats the 1020 in performance. No stuttering at all, and very responsive screen. Will see how it fares in a few weeks when I've used it more, maybe it'll start doing minor stuttering after more use.
You can change the screens colour profile in settings to more muted tones.