I first saw the Ergo-Active thing and went... wha?
Tried it on, and was like... WOAH.
Scrambling sketchy 4th class with a pack on can be scary, but this thing moves with you like no other pack can, keeping your center of gravity from dramatically shifting and throwing you off balance. The material feels super burley, and the fit is very comfortable on my back. The 45 has a very long torso which was kind of strange on me (I'm 5'10" with weirdly long legs and a shortish torso), but the 35 feels awesome.
Big fan of the rope strap/lid, holds a rope great.