Seems to work quite well in very bright sun.
Used it only twice really before the daylight hours became too ridiculous to bring it along.
With some effort you could really increase the life of your battery packs on a long trip, as long as you have some sun.
Just have to work on setting it up and aiming it right at the sun. But far less effort than trying to bring another battery pack.
In full sun it charged quite fast.
I have a 20000 battery pack, I can recharge my phone about 4 times on this. I think I could easily charge that pack at least half way on a sunny day.