The GM has been good so far on my 6 Plus coming directly from 8.4.1. The only odd thing, it was really warm when I unplugged it from the charger this morning. I realized it was still uploading/syncing pics to iCloud Photo Library. For some reason this is going excruciatingly slow. Other than that, I haven't noticed any issues.
My iPad Air 2 that was running PB3 however has been a complete disaster from beginning to end. On first boot after the restore, it was asking me for my iTunes store password and wouldn't accept the password for my Apple ID. Had to quickly dismiss the prompts and opt out of everything during the initial setup in order to get into the iPad. For some reason the app restore wouldn't complete and I can't do things like sync photos with iCloud Photo Library until the restore finishes. Now I have stuck update notifications on both the app store and the settings app that won't go away.