Not noticing many issues at all. Quick reply already works for Whatsapp, pretty great, I think because the functionality for it already exists in bluetooth add-ons and Android Wear?
Battery life remains to be seen, I did a fresh and install, so that is eating too much power to give a fair estimate. Will see soon. Perfrmance on the 6P is great, not noticing any slowdowns other than ones related to downloading an installing 160+ apps.
All the new functionality is heaven and it doesn't even have the multi windows working properly, obviously. Android N is where Lollipop finally becomes an adult it seems.
Love the quick toggles, new notifications and the bastly improved settings screen.
Also, the restore of a backup on a fresh install is finally as it should be. Icons appear on the home screen as they were and it starts downloading every one of your apps, showing a progress notification to tell you how many apps remain. No more random downloading of apps, stopping for no reason, and question marks left. Good stuff, long overdue.