The 857 is a great radio, especially for mobile. I know a couple of guys that have them, and love them. They run great from battery power. Some HF rigs will shut down when run at higher power on 12v batteries.
All that said, the 857 is a little heavy for backpacking. I think if I were going to play with going out in the woods w/ a radio on my back, and I didn't want to use just morse code, I'd look for an 817. And a little manual tuner. And a homemade wire antenna. (Really, really easy to do, and works as well as anything you can buy.) And an android phone or tablet with PSK and a homemade cable. With 5 watts and code or PSK, you can talk around the world on HF.
But if I were looking for a mobile or emergency use HF rig, the 857 would be at the top of my list.