In reply to 007:
For me, buying a rucksack is much like buying a pair of boots. They all fit slightly differently and the key is to try them on (weighted) and find what fits you best.
I also find that I hate carrying over- or under- stuffed bags so despite my resolution to buy one good one, I know have 5 or 6!
FWIW, my most-used sac is a Crux AK37 - the floating lid version. It comes in a couple of back-lengths. I find it super-comfy. It's a very light sac, but it has achieved that by being minimalist - it has axe loops but not much else on the outside and apart from the lid, just a single big pocket inside. I have added what I want on the outside with bungee, which suits me better anyway. The material feels kinda flimsy, but it has lasted me years, including a couple of trips over the Cuillin on the rough rough rock.
When I buy sacs, I look for narrow bags (no side pockets) so they don't restrict arm movements too much, a solid hip-belt, and something that doesn't hinder head movement too much when wearing a helmet. This is the reason I like the floating lid on mine - I can move it out of the way or remove it entirely when climbing with it.
