i have my XZ1 with kitkat on it.
it's not that big of an upgrade... most noticable is better camera algorithem, so a little less noisy pictures, and little better night shots.
there is a MAJOR sound issue with kitkat, sound is coming from earpiece instead of louspeaker, so people don't hear thier alarm and incoming calls...
sony promised an update this week, other that that ram handling is bad like in 4.3, it's getting low all the time and the device starts going slow and closing apps....
not much of a promise... and we didn't get any cool features from the Z2 like tap to wake or smart call handling...