I'm using the G2 for a bit more than two weeks now, and so far I am reall impressed by the battery life.
With my old One X I wasn't really a fan of playing, because you could literally watch the battery drain, but with the G2 I came back to playing Real Racing 3 and Angry Birds Go.
My experience is that battery life while gaming also depends on the game (of course) and the screen brightness - battery drains faster while playing Real Racing 3, but playing for an hour or so doesn't drain the battery more than 15-17%, with Angry Birds Go it's even less (approx 10%).
This leaves me with Screen-On times of 5-6 hrs > 60-90 mins of playing, emails, FB, internet, photographing, GPS tracking,... (settings: WIFI on, brightness > Auto, Data on, GPS on)
The back buttons look solid and I don't really think that they will cause a problem or fall out. But I guess that also pretty much depends on how what you're doing with your phone and how you treat it