I've been using the Mophie Powerstation Duo since this summer. I actually bought it on the way to the 2013 Boy Scout Jamboree to replace the portable battery I'd forgotten to pack. Picked it up at a Best Buy for the MSRP of $100. It's a 6000mAh battery for USB-powered devices.
If you know me, you know I'm addicted to my iPhone. The problem is that I use my phone so much, it's not uncommon to need a second full charge during the day to make my phone last all day.
Since I use a LifeProof iPhone case, I can't use a case with built-in battery power. That means I'm stuck carrying a USB power supply with me almost all the time, because I hate having a dead phone.
I like the Mophie battery. I've used it regularly over the last 9 months, and it still appears to hold a charge without issue. On average, I get 2 full charges of my iPhone, or a full charge of my iPad off of a fully-charged battery. That's while I'm using it, so it's by no means "perfect" conditions. The battery takes ~4 hours or so to charge completely from empty.
The battery is small enough to fit in a coat pocket or my work trouser cargo pocket when I want to travel light, and it usually lives in my "satchel" that I carry to work with my iPad and various charging cables. It also appears well designed and durable. I've dropped the battery a couple of times and it's still working fine.
The 2nd USB port is very handy for charging my iPhone and iPad at the same time, or charging a friend or co-workers device. The battery charges off of any standard Micro USB cable, so recharging isn't difficult. Out of the whole product line, it's probably the best combination of capacity and compactness in their line
If you're on GovX, the device can be had for a decent discount, but I also don't think the device wasn't worth what I paid for it. If you're looking for a portable battery for your digital devices, I think this is a good product.