I went on Amazon and got 2 hyperion 1800mah and a wall charger for $25 shipped, came super fast even with super saver. I power cycled them with an almost 8 hour video over wifi on youtube with brightness maxed and nav on, first cycle life was pathetic, 2 hours, after 3 cycles I have 4.5-5 hours.
the video is some kids black ops record for C4 kills in 7 or 8 hours. just left the phone on silent while I worked and let the video play, waited for beep #1 of lower battery, and then held out for the critical where screen brightness is capped, turned the phone off swapped batteries and stuck it on the charger)
I keep one spare in my work bag and one in my glove box. Did that with my EVO 4G also. The Epic has much better life and doesn't turn into a hot plate like the evo always did...
4 to 5 hours is pretty good. I get around 3 hours of battery life playing videos with the hyperion battery. I also bought some generic cheap 3x battery for $12. Those get less than 2 hours of video playtime.