Deff worth the update !
So far with the latest open eur rom ( 20d ) i've noticed changes :
Faster auto-focus (but still no 4k recording on stock app)
Audio kitkat bug is fixed (there was hissing on the left channel, now it's just like on jb )
KnockOn works everytime and is a lot faster in actually waking the screen (the delay between tapping on the screen and screen waking up is really small now) and with the latest kk update, you get KnockCode which is awesome !
Ui feels a lot smoother
Now, if you root and install xposed + app settings, you can enable immersive mod in every app ( immersive mod hides the notification bar and navbar, so you can fully enjoy your 5.2" screen)
And if you use emoji, the messaging app now doesn't change your sms into mms ( if you put an emoji on jellybean, the sms will be sent as an mms... atleast i had that on D802 )
I haven't found any other bugs