I've had my GG 3500 for maybe 4 months... It is considerably better than stock. Certainly not twice the on/usage time though. And, I sorta think it's already biting the dust.
I also had bought an OEM BH6X. After comparing it, the stock and the GG3500 a few months, the BH6X seems to run a nice amount over stock, certainly not quite as much as the GG. But, it fits better (the GG always had a loose door and the battery fell right out), and the phone is obviously slim with the OEM style battery and the door doesn't constantly come off in my pocket like the GG
I bought another BH6X and stand alone charger and swap them back and forth. I used the GG heavily for a while, but feel it just wasn't worth the hassle of the size, door issue etc.. I keep it around for all day trips as a back up emergency source.
Just can't say I'm that amazed at it, although I reallllly wanted to be. Good price on the GG, as the BH6X is 33 bucks on ebay. But I think the cells in the GG are somewhat slightly questionable. I will probably grab another BH6X so I have three and that should be safe. lol...