Has anyone used these yet? I just got one for xmas. I was going to bring it on the AT but people are saying not worth it... but are they talking about this newer model or past ones? Backpacker says this one is much more efficient.. is that so? I am more than happy to leave it if its not going to help.. but I would like to be able to charge effectively in some way.. I am going to carry a cell phone, mp3 (AAA), GoPro camera (got a spare battery).