I took the Gossamer gear Pack on my John Muir Trail hike (not the first time used - about it's 5th outing). It performed superbly. However, nothing is perfect in the "lightweight world"! But, Gossamer Gear have just about got it perfect.
I have recommended to Grant at GG, along with others (see Brian's Outdoor Blog - link in right panel) several "fixes":
1. Go back to a convention "s" narrower, padded shoulder straps. The wide straps rub on neck sometimes, the internal padding slips and overall it feels "strange" on the outer shoulders.
2. The compression straps at the top of each side are useless - lower them or dismiss them.
3. The strop that the lid strap threads through is too narrow for the buckle and too low - it impedes entry to the top of the back mesh pocket.
4. The fabric , whilst bomb proof, does not perform well in rain - seems to "soak" through quickly.
5. The velcro secures for the internal back frame are too small
Having said this, the pack performed wonderfully and I got many comments on the trail as to the it's look/design.