I've got an Asolo packsack, the 'Encounter 70', and it's the best pack for photographing I've ever had. The best feature is the front loading main compartment, accessed by a large inverted U shaped zipper that just fits my 10" wide x 15" long x 8" deep camera bag. No more dragging the camera bag out the top of the pack!!!!
The top compartment is more than large enough for emergency supplies and the bottom compartment easily holds raingear for myself, the camera and a focusing cloth. In winter the main compartment has enough wiggle room to fit in extra warm clothes. At the very bottom of the pack is a small pouch housing a rain cover that fits over the pack...a small feature with huge potential during the fall monsoon.