They aren't worth the money or the weight. You are almost always better off bringing the equivalent in spare rechargeable batteries. I've written an article on it:
Works well on sunny days if you adjust it to point directly at the sun, and it charges in 6-8 hours.
Is it practical even then? I'd gladly carry a few extra batteries so that I don't have to sit around camp and keep adjusting the panel every hour.
$100 isn't practical for most people either.