I know I'm awa to get shot down in flames with this post!
I was looking recently for a lighter expedition sack, as the 60 litre Vango sack I bought of ebay a couple of years ago, weighed over 3kg! I looked at a lot of sacks, but ended up buying a Berghaus C7 with this back system with a pivot in it, and the main body is waterproof - handy! The down side, is that my new rucksack also weighs almost 3kg. This weekend though, I was carrying an extreme load - not only the obligatory extra winter gear, but more food than usual, and (the killer) fuel for the fire - must of been 18kg or so when I set off, and yet it felt lighter than my usual 10kg, because the pack fitted so well, and I wasnt fighting against it all the time.. I'm a convert - comfy is the new light