I updated my k4 note to marshmallow. here’s my honest review.
these points are what I have come across so far in 2 days
Pros - 1) little bit app opening is fast now.
2) No bug at all, wifi camera fingerprint all working perfectly.
3) battery performance is still same as was in lollipop.
4) Now you can register more than 2 fingerprints.
5) New feature where SD card can be used as internal memory.(requires UHS sd
card for better performance)
Cons-
1) Now it draws some more RAM - with lollipop I was getting around 1700-1800 mb
free but now I get only 1100-1300 mb free with same amount of apps.
2) Network speed info on notification feature now removed.
3) Marshmallow doesnt have "default write disk" feature like one in lollipop.
So it requires application level setting for default writing in sd card.(which is not
available with current whatsapp). so whatsapp media files downloads in internal.
However Camera app has location setting, so new pics and videos goes in SD.