I hace the Arc’Teryx Bora 80 which is roughly 5000 CUI and wieghts in at a beefy 7.2 lbs. The suspension is incredible for heavy loads but is this too much pack for an AT thru-hike? What size pack do must people carry and is the weight a critical issue on chosing a pack? Am I going to end up being pissed that I have this pack after the first month of hiking? This one has worked well in the past for shorter winter hikes.
Olsen
There have been lots of discussions on packs under the Gear Forum. Check them out 
Jim Deane, Rebel with a Cause !!
Jim Deane
Use the smallest and best suspended pack you can find that fits YOU and is super comfy. This sounds lame but take all the things you want to thru hike with and go to the outfitter and start filling packs that you like. Pick a week day when they aren’t very busy as this may take a while but it is the best way to buy a pack. Gregory Foresters, my Gregory Thru-Hiker, Mountainsmiths, home made, a Kelty Suuuuper Tioga extrenal frame, we saw it all on the trail. Just make sure you have the gear list as close as possible, with five days of food, a quart of water, everything like you’re going camping. Go in the corner and fill’er up and walk around the store with each pack for an hour-yeah, you look lame but you’ll also get style points for buying it right. I walked an outfitter in Hanover, NH for two hours in a pair of boots with my pack on. The sales guy tailed me like a hawk. But they fit.
Ease of water bottle extraction, water tightness, no extra zippers nad crap to add weight, look for function not hotrod looks.
Bushwhack