In reply to The Ex-Engineer:
A key difference between the Arcteryx and its obvious competitors is the fabric, which, while more durable, is less comfortable against the body in terms of breathability/heat generation/sweatiness. This may not be a big issue to many, but to me it's a major factor in gear choice as my skin reacts badly to heat. For others durability may be a higher priority. To me, the durability just means being cursed with an uncomfortable pack for longer!! The Patagonia Ascensionist is far superior in this respect, as well as taking loads more comfortably and with greater stability. The lid design is a dream at belays for whipping out the jacket/water as the mechanism is so quick and the opening so wide. Great design.