Just pre-ordered an extended battery case:
PowerGlider External Battery Case for HTC One (M7)
Was considering getting the MOPHIE version:
mophie juice packŪ HTC One Extended Battery Case
However,I'm liking the design of the i-BLASON case,as it appears to be slimmer & a better top-cap design,which appears to be physically tethered to the body vs. a removable top-cap w/the MOPHIE model.
The battery capacities are identical,adding an extra 2500 mAh to the on-board 2300 mAh w/the HTC ONE.
I ordered it through AMAZON,as it's priced $10 less than @ the mfgs.website:
Amazon.com: i-Blason PowerGlider For 2013 HTC One (M7) Smart Phone Rechargeable External Battery Glider Full Protection Case - AT&T, Sprint, Verizon, T-mobile (Black): Electronics
Shipping on MAY 30,w/an ETA of 6/4/13.
Will post an initial fit-finish review to be followed w/an in-depth report on battery life soon after.