I have a FuelRod that I bought at an airport for $20. It's the same one you buy in the park only $10 cheaper.
It's convenient with the swapping, but it's not the most efficient charger. It's more of a trickle charge, and it will only charge about 1/2 of my iPhone. If you're actively using your phone, it won't charge your phone. It just stops the battery from going down.
I'm happy with my choice to buy one. I travel all the time and go to the parks a lot, and I'd forget to charge something like the Anker one.