Marshmallow seems to work well for me. I have a Nexus 5, and I used to have KitKat (4.4.4) on it. Battery life was getting worse at the end, even after applying the various tweaks (like changing screen brightness, turn off Location, use 3G instead of LTE, etc.). After wiping and flashing to Marshmallow, I applied the same tweaks, except for 2: Location was kept at "Device Only," and LTE was on. Despite that, it feels like battery life has improved. We'll see as I keep using Marshmallow on my Nexus 5.