1:16 p.m. on September 8, 2012 (EDT)
I'm in the camp that understands that nowadays you don't need a big pack to carry big, heavy gear because smaller, lighter gear is available. This is especially true for less than week-long hikes, which is pretty much what the AT is, a collection of short hikes between lots of resupply points. It is not expedition hiking, not a Lewis and Clark kinda' thing.
It is also my understanding that the AT is in way too many places a poorly engineered trail in terms of slope (goes straight up the hill, which is never much fun, and then straight down the hill, uselessly even less fun, or so I have read) and that means it is a struggle not improved by carrying a heavy load. Rumour, of course, never been on it, don't intend to ever be on it, not my kind of trip.
That being said I used externals for many years because, well, there weren't any internal packs. I never had a problem that mattered much but then my walking was all "PCT" sorta' stuff. Rational slopes, mostly. Walked north to Yosemite out of Cedar Grove in '65, solo, wool clothing and combat boots kinda' thing, no-name (I'm sure it had one but I don't remember what it was, long before marketing took over) external pack. Great trip. Mostly good-enough switchbacks, unless they were blown-out or MIA, rational engineering generally. No trail at all that I could find in some places but I was just a kid and my skills then were not what they are today. Pretty much nobody up there, taking my life in my hands crossing heavy water and other terrors as well. Heavy load, lots of work, would have been way better using our modern equipment.
It was the most memorable walk I ever took.
I don't even own a frame today. I would if I was going expedition/no resupply for weeks on end into the bush, if I needed to haul significant wood to a base camp, move meat, that sort of thing. Too old for that now, never gonna' do it again, but I have done it and were I to do it again I absolutely would take a frame.
But not on the AT.
You can use whatever pack suits you of course, HYOH always, but I would advise that you get your load weight down as low as possible and spend some time checking out internals. You don't have to spend a lot of money on the bag. I can do a couple of weeks in good weather with a $110, 4 pound Kelty Lakota. There are other perfectly serviceable bags out there that will work well enough for little money. Of course, depending on what equipment you already own, you may have to spend some potentially serious money on the gear that you put inside whatever bag you select.
Whatever, equipment is pretty cheap when you consider the benefit of walking around in the weeds.
Decrease the suffering, increase the joy and always practice simplicity.
Make it a great experience!