I just got my industry deal Gregory Z30 in the mail today to try it for this very specific reason: my desire to have almost no weight at all on my shoulders, breathable back panel, and to comfortably carry a full 5-7 days worth of gear & food plus 2 liters of water. I will be comparing this pack right next to my new 2012 Gossamer Gear Gorilla (Gorilla around 24 oz. (with sit pad) vs. 42 oz. for the Z30). Here is my reasoning behind comparing these two packs: (Also wondering if for the average backpacker -- an almost Doubled Price Tag of the Gorilla isn't an immediate and deciding factor for most) Was also considering including the new Mountain Hardwear Fluid 32 in this comparison, but as of yet haven't picked it up.
Gorilla: Very durable, light, comfy, gets pretty hot against my back in summer (all these style packs I've owned - MLD Burn, MLD Exodus, ULA Circuit, GG Mariposa, all don't breath with the pack sitting right against your back). At weights of around 20 lbs. I start to notice the weight on my shoulders and neck stiffness. Expensive but excellent company with pretty much the most comfortable shoulder straps on a smaller volume pack that I've ever tried. Excellent addition of hipbelt pockets. Very good pack compression.
Z30: (Have yet to field test - but these are my around the house first impressions) Very durable, very comfortable, noticeably more ventilation along the back panel., can put almost no weight on the shoulders at 20-25 lbs. via its very comfortable (albeit heavier suspension system). Rather inexpensive in comparison, better feel of the load transfer to the hipbelt. Almost useless hipbelt pockets.
NOTE: For a comparison I'll throw in my impressions of the MLD Burn (used for about 1,700 miles last year on my A.T thru-hike)
MLD Burn: (about 10 oz. if I remember correctly) A great pack overall. Didn't quite get the weight transfer off my shoulders (with is NOT the design of this pack, so it is not the packs fault). I found it to be magical for loads under 18 lbs. and with a base weight of around 8 lbs. Absolute BOMBER pack -- your children will be using it in 20 yrs. when you feel like upgrading.