Has anyone looked at the monoprice units? Specifically, the
For only $11.48 each when QTY 50+ purchased - Micro USB Backup Battery Pack for Smart Phones, Cell Phones and Cameras (2800mAh) - Black (REV. 2.0) | Back Up External Battery and Charger
Seems like this (version 2.0) unit is getting great reviews. Smaller and more portable than the bigger units (but less charge). Seems like something you could pocket if you're planning on being away from a charger for awhile. One feature I like is you don't need any extra cables. It charges from the same microusb that you'd use to charge your phone, and it looks like you could just charge through (charge your phone connected to the battery connected to the wall charger).
Not a user, just an interested customer.
The long term question is more about what happens when the internal battery starts to lose capacity. An "easy" swap, or major brain surgery...