Well I've purchased one. In a week's time when it's delivered, I'll be the owner of the G4. Black leather model.
Really really really hope it gets me better battery life than my current S6. If it doesn't, I'll be very disappointed.
I think you can (note can) get better battery life with it, however as with most phones I believe it will vary depending on your individual phone, habits, and how it is set up, and as I have found on mine, getting enough cycles on the battery.
I've found that all of the Super bright white backgrounds plastered all over Lollipop plays havoc with everyone's battery life. The bigger the screen, the worse it is. While we each have our preference on how our phone looks I think this issue will always be the cause for the wide discrepancies we are seeing with this phone. Not only this but with the automatic brightness settings. I think people are going to have to find the right balance, not just use what is the default settings. I would imagine that it is similar with the g6 in this regard too.
Turning off apps that are constantly syncing that you don't use is a biggie too. While all of the neat things the OEM's load on our phone is nice, not all are doing the battery any favors.
One of the nicest bonuses I have found out about this phone is it's standby time. On mine it is better than any phone I have ever owned including my m8. I will have to just let it sit for a few days to really see how much better, but all indications leads me to say that it is double of the m8.
I've owned many Android phones over the years some bad, some good, some great, but this one does seems to do everything I like better than all the rest so far.
I hope your experience is the same.
Posted with my awesome LG g4