I just purchased a new pack, the Osprey Aether 70. I have to say...WOW! Having a pack custom fitted makes a world of difference! The Osprey hipbelt was heated and molded to my shape so the whole thing fits like a glove. I loaded the pack with about 30 lbs and could barely feel it!
My old pack, a Kelty, obviously didn't fit me correctly. When I lugged around 20-25lbs, it felt really heavy and my shoulders and back would get sore within just a few miles. I could instantly tell this new back fit and would give me much less grief in carrying weight.
For everyone here who has mentioned getting a custom fit at an outfitter for a pack on the various threads, thank you! You are spot-on!