I'm in the UK and gave in to the 2 day old update on Saturday, only because I heard KK has addressed the WiFi unstable issue. I was determined to stick on 4.3 before that.
It's early days so I may run into issues and can't report on the SD card yet.
However all went very well. Apart from the Notification Bar icons in white (I like it now with a white phone) everything on display is exactly the same.
I'm pleased the menu system I took ages to learn is still there. All apps that I have tried work fine, though the way some display drop down lists etc is different (banks/others). They must have been updated for KK before I installed and changed seamlessly.
2 minor things: initially I was not getting the public wifi unsuitability fussiness in the same places, but that signal too weak/unstable warning has re-appeared a couple of times, but less often. I carry 2 phones around and my GS2 never has that.
I also had an issue yesterday with syncing when I kept getting notifications to log in to Picasa which I had never done. Sorted with great Mods help, but it does seem after install it ticks all the (sync/other) settings boxes you may have unticked before.
So I can recommend it if you have a factory sim free one like mine as there was no drama. The same apps and other files are on the SD card. I haven't tried to install new ones there
Does it cure the fussy wifi, maybe not completely, but nothing negative to report.
At the same time 4.3 may work as well for you (battery and efficiency are supposed to be better on 4.4 and it seems good but too early - 4.3 was fine anyway)
So for me once I solved the sync everything is fine, exactly the same phone, and apps and S-Planner look better.
I think in time app developers will push you towards KK so now I know it's fine I am OK. I was complaining right along with others beforehand that they should not have changed SD card access, so I may encounter issues, but so far all is the same.
Nobody knows how much I don't know. Via my Note 3 on AC.