I have been running Windows Phone 8.1 Update for a couple of weeks now and have to say I like. I have not suffered the poor battery life others seem to have suffered. Maybe this is an feature of the Nokia 820 no needing as many firmware updates from Nokia (which aren’t available yet) note having such power hungry features as the larger phones.
The only issue I have had is that I lost an audio channel when using a headset. Initially I was unsure if it was a mechanical fault on the headphone socket, but I checked the headset was good, it sounded as if the balance was faded to just one side as you could just hear something faint on the failing side. Anyway as often is the case in IT, a reboot of the phone fixed the issue.