As for 18650, stlpaul described it accurately. Being widely used, standardized, cheaper (as money You need to pay per mAh), easily available in plentora of types, etc, benefits of using them over standard "phone" flat batteries are gigantic. It's just that they're little harder to fit, due to cylindrical size. But, even having this restriction in mind, they still supply more mAh per square cm. (even considering a "loss" due to empty space between cylinders). Only one drawback, is that You can't go lower than width of one single cell, i.e. it have a minimum thickness much bigger than flat battery. But, for people that already use Mugen covers for dual-batteries, it should be a problem - as long as it still fits into pocket I would happily trade few mm of additional thickness for having 6000 mAh + additional mod space + guarantee, that my batteries won't ever disappear like scuds + cheaper batteries as opposed to: 3000 mAh and lack of all mentioned benefits. /Estel