I got one prior to my 18-month trip across the Americas. I used it to charge battery packs that then get used to charge my cycle computer - particularly in a few gaps of multiple days with no charge options e.g. Dalton Highway or a gap or two in Peru and Argentina. As it turned out, the wiring was fragile and broke in Alaska. At that point, I could get there by charging a battery pack from wall outlet and using it for the cycle computer. So it was a nice idea but not one I ended up using as much as I expected.
So I see them as interesting luxuries, particularly if you are in a place where you are likely to be able to charge most nights.