My son bought one of these and at the end of the day he has 15-20% battery life left, depending on usage. And he even plays games on it when bored (2048 works quite well, apparently)

He doesn't use ambient mode, and for those rare occasions where the screen doesn't wake up on lifting, a button tap seems to not present too much annoyance.

He does tweak it and his Galaxy S5 a lot - latest and greatest ROMS, updates etc., and says it is very straightforward to mess around with for incremental improvements.