I'm a big fan of spare batteries, but that Mophie pack looks ... way (2-3x) overpriced for what it is ($100 for a 6000mAH battery w/ 2 USB ports, 2.51x4.52x1.03in, 12oz).
In my bag for the past couple years, I've been carrying around this iSound 16,000mAh battery which supports 5 USB devices charging at 2.4A (it's slightly heavier/bigger 5.8x3.2x1in, 14.6oz) and sells for about $70 - the more comparable 8000mAh version is $50 (3.5x3.5x1in, 8.2oz). While Amazon shows reviews giving warnings about the quality, we about 30 of them in the office and have had maybe one stop charging over 2 years.
A directly comparable device w/ good Amazon ratings (but that I haven't had personal experience with) is the Satechi 10000 mAh (actual capacity reportedly about 6000mAh). It offers 2 USB ports 2A, 2.3x5.7x0.8in, 7.4oz and costs $60 or even better yet, the Monoprice 8000mAh - 2 USB 2.1A, 2.3x4.6x0.8in, 9.1oz) for a ridiculously cheap $28.48.
(For those that don't know about Monoprice, they're awesome - they do testing/vetting to provide bulletproof house brand parts (started w/ cables but they've been expanding) at dirt-cheap prices. The also do same-day/next-day shipping in California.)