Might be late to the party, but I returned two G6's back to T-Mobile due to wonky rear power buttons. The first one was, like yours, woefully unreliable... would work maybe 25% of the time. The second one was just as bad, but this time the button at least felt busted (it felt a wee bit wobbly, like the control disk on an old Intellivision controller ... ok, I'm, dating myself there)
The THIRD one... well... that one did the trick. The button didn't feel like it was hanging on for its dear life AND it did its intended job every time.