Sharing my experience with kitkat update on N3.
Frequent random reboot after the update. This was happening at every few minutes if the phone was under heavy usage. factory rebuild several times and clear cache didn't fix the problem. It has gotten annoying to the point that I have to switch to a backup phone. Either internal ic or overheat problem.
Sent it to Service center. After 3 days it came back the officer said they couldn't detect the random hang issue, and reflashed the firmware. Obviously It came back unfixed.
Waited couple of weeks and saw the new firmware update. Patched it and problem persist. Somewhere in hwz I read that this could be fixed by replacing the mb.
Sent to service center second time. Took the effort to explain to them I want the motherboard to be replaced. Customer service wrote down all the details (random freeze, second visit, need motherboard replace etc).
3 days later went back and again the officer told me they couldn't duplicate the issue and did a reflash of firmware. The invoice has a BIG QC Pass chop. I wonder what kind of QC work did they do behind the wall. I did a test on the spot and showed to them the phone hanged and rebooted. Your fix didn't work!
Here came the best part: The customer service took my phone, navigated to the RAM screen and calmly told me "Sir, you see this RAM status. If you run a lot of apps, and it maxed out all the memory, your phone will restart."
Despite my best effort to be reasonable after wasting the time and effort, this last sentence did it in. Being ignorant is one thing, taking your customer as an idiot is another. How can Samsung allowed for such idiotic reply for one of the flagship products.
Anyway, after giving him my 200 dollars worth of view, I have asked for the manager, re-emphasized the whole situation, and after yet another 3 days, the phone came back with motherboard being replaced, and problem no more.
This much I have to say about Samsung after sale customer care.
Sorry for long post.