I've been carrying a Kifaru original Longhunter 5500 ci bag for many years. 2 years ago I upgraded the Duplex frame to the new Ultralight Hunter frame and so far have loved the weight savings and the same heavy load capacity of the system.

I would definitely try to get the pack in my hands first and then put 80-100 pounds in it and see how it feels and carries. I tried a pack from a well-known maker a couple years back at the SCI convention. I knew the fellow working the show and I know their company builds quality gear. With no weight the pack felt like putting on a shirt, very comfy. I then asked for 100 pounds of sand bags. With weight, the pack felt like a I had cinder block in my lower back...no go. It may sound extreme, but when you are 8-10 miles in the wilderness and have a bear or elk down and your option is 1 trip out, how that pack carries is very critical.
I realize not everyone is carrying those loads, but try it before you buy to see how it handles the weight. No pack is fun when north of 100 pounds. But, I can attest to the fact the Kifaru system will allow you to survive the ordeal:)