A pack is the most important piece of gear on a backpacking trip, and I thru-hiked the Appalachian Trail with the Gregory Z55 (2010 model). This pack worked wonderfully for me.
The L-size is actually 61 liters, which fit all my gear perfectly -- sleeping bag/pillow in a compression sack went into the bottom, and it slipped right in with no problems, despite the curved frame.
The adjustable hipbelt was helpful since I, of course, lost weight on the AT. The entire hipbelt and harness system was quite comfortable on me once properly adjusted. And the suspension easily handled loads up to 40 pounds. Beyond that, there was a nice creaking with every step I took.
Most thru-hikers wear out their packs on the AT. Not so with this one. It's ready to go again.