In reply to zimpara:
Unlike some people in this thread I really do find waist belts incredibly useful if not essential. When I go cragging (i.e. anything less than an hours walk-in) I am often carrying probably far too much stuff because a) I don't really care b) it doesn't really matter and c) I like stuff
So when carrying 15kg+ (at a guess; I have spreadsheets for rack and non-rack but haven't combined them), if I don't use the waist belt I find it _rapidly_ uncomfortable on my shoulders (as in within 5-10 minutes). Basically I like as much of the weight as possible to go on my waist.
I use a Montane Torque where the standard padded waist belt is completely removeable. It also comes with a webbing belt which (again unlike some people) I find really nice when climbing with not very much weight in the backpack (quite easily 5kg+ given the weight of the bag, rather excessive FA kit, warm jackets not currently in use, 1l - 2l of liquid, lunch etc). It doesn't get in the way of the harness and holds the bag steady while climbing. Again I really don't like the weight on my weak, weak shoulders if I can avoid it.
Anything mountainous I carry rather less :P