My Tesco MotoG offered the update late last night.
What's new - really not that much - transparent status and nav bar and a new settings icon are the most obvious UI changes, new stock Email App along the lines of Gmail, new Dialer app, new Gallery app, ability to switch easily between stock and user installed launchers from Settings, printing from the device, some extra options in Developer Options including experimental ART runtime rather than Dalvik and 'Geeky stats about running processes' (great name for a settings entry). Repeatedly touch Build Number in About Phone to unhide Developer Options in Settings.
What's broken/annoying - user apps can't access app battery/cpu/wakelock stats without root anymore, doesn't seem to be native support for USB drives to be mounted via OTG - *sigh* - very bad to offer updates to drunk people on a Friday night!
Time will tell if performance & battery life is better/worse than the original 4.3.
Edit - improved Antutu benchmark - 17611, compared to 17200ish on 4.3.