My experience with stock marshmallow so far: upgraded yesterday, took about 20 minutes of installation; i didn't do a factory reset. Smoothness is much improved, antutu increased from around 79000 to around 84000, average free ram doubled from 500 mb to 1gb, overnight battery drain decreased from 8-11% to 5%. Nice to have out of the board raw support instead of having to use another camera app. No problem whatsoever with previously installed apps, i thought i would end doing a factory reset after the ota due to the dirty flash but it seems it's buttery smooth and without bugs even without it. It was a long wait but i'm happy with the upgrade! I will update this with my total battery time and sot after a full charge, previously on 5.1.1 on average battery would last me about one day and half with 3,5/4 h sot, and i could achieve 5,5 h+ sot with intensive use at the expense of total battery time. I hope my battery with my use will now last 2 full days. My build is G920FXXU3DPBG