I shopped around quite a bit before buying this solar charger. After using it for a week while camping in the New Mexico wilderness, I have the following to report.
(+) It does indeed hold a charge for a long time. I charged it up to 5 flashes (full charge) via plug-in power beforehand, and it was still full charge when I needed it 2 weeks later.
(-) I couldn't imagine any better condition to charge from the sun, than on top of my car's hood, under a clear New Mexico sky at ~10K altitude. Even then, a full day's exposure would only get about 60% charge (3 flashes out of 5, starting from 0 or 1).
(+) It appears to be water proof/water resistant. I decided to leave it out on my car's hood even during the rain. It still works!
(-) I experimented with charging it on the dashboard of my car at home. Even after sitting on the hot/exposed dashboard for several days, it didn't appear to get any charge. This might be due to my polarized front windshield -- not sure, but after my camping/raining experience, I won't feel bad about leaving this exposed on my porch, from now on.
Over a week's use camping, it did manage (barely) to keep my iPhone charged up. So, all in all, this isn't bad, but it isn't perfect either. It would be even better if there were some real time indicator showing if it was getting a good charge or not.