I was really worried that this was going to be a waste of money, but it was on sale and I thought, "Why not?" I was going camping, in a yurt, that I had to hike to, in snowshoes, 2.5 miles just to get to. Keeping my equipment load light was a must, but I needed my tablet both to a) read books, my principle leisure activity, and b) get some work done while on vacation to keep my boss happy.
I preloaded the charger in a wall outlet, that took about two hours. After that, I just left it in a sunny spot. It kept three smartphones, a sat phone, and two tablets active, under heavy usage, on the side of a mountain. We didn't have running water and had to poop in a hole, but dammit we had working apple products! When my friends and I left the mountain, the charger had 4/5 bars still!
I love it. I keep it in my back pack (yay being a scientist, backpacks are totally professional!) Whenever I need a quick charge for something, I plug it in and boom! charged device. Just leave it in a window, even a tinted window, to charge itself.
I highly recommend this product. It's sturdy, performs exactly as advertised, and is reasonably priced. It's useful on both the side of the mountain (though it can do nothing for you when you now-former-buddy drops the TP in the snow), as well as in the busy life of a professional on the go.
Update: So I read some of the 1 and 2 star reviews, and I have comments on the comments. To get best use out of the device you should:
tl;dr - read the instruction manual
1) Pre-load the charge in a wall outlet. I charged it once on AC/DC and now just top it off in the sun. No problems. People seem to complain about how long it takes to fully load the battery in just sun. The device's instruction manual tells you to first fully charge it from a wall outlet - follow the instruction manual. I did and I have had zero problems.
2) Keep it charged. Once again, the instruction manual says don't let the battery empty out. Also, who doesn't know that if you don't use a modern computer device, e.g. smartphone, laptop, etc., and just let the battery run down and leave it off that its battery is damaged? Same applies here. When it's not in use, just leave it on your window ledge. When I remember, I stick it in the window at work, maybe once a week, and the battery is always fully charged or close.
3) It comes with short little nubby cables to charge things with. I've never used them. The powerpak has a USB and a micro-USB female plug. Use your own cables. It doesn't come with a nubby iPhone 5 cable, but that's not listed in the description or shown in the product pics. Plus, I repeat, use your own cables. I tested nubby versus original. All devices power faster and deplete less energy from the powerpak when you use their native cables.
4) It doesn't pull in charge as fast as most smartphones and tablets will drain it. And it isn't designed to work from an empty charge (instruction manual). Keeping the battery in sun while you charge your device will mitigate charge depletion, but the output is still greater than the input. Act accordingly. Common sense, don't leave your device plugged in once fully charged. Let the battery fill up again. Rotate devices through the charger unless it's an emergency. It's a solar cell, it doesn't work as quickly or as strongly as chemical redox charge batteries.