Got a Circuit in 2009. On the first couple of trips it felt amazing when I compared it to a traditional-style ten year old cheap workhorse backpack that I used mainly traveling (non-wilderness) around the world. Functional, comfortable and light.
After a couple of more short trips the Circuit felt too big, unreliable and too complex with all the hundred long straps. The carbon fiber stay broke (don't know how or when exactly), both of the sewn-in cord loops that held the side pocket cord-locks came off, a stitched seam started unraveling somewhere...
Took off the hip-belt, back panels and the carbon loop, fixed the stitching, cut all of the extra webbing. Result: Awesome! Really nice for shorter trips. Started liking the pack again.
Attached hip-belt and the back panels back on. Packed two weeks worth of food, fishing equipment, etc. Max pack weight maybe 18 kg (40 lb). Worked perfectly the entire trip WITHOUT the silly carbon fiber stay (20+ mile days). Really liked the pack ever since that trip.
Now, after some 500+ miles with this pack, my opinions:
* I trust the pack, although there were non-catastrophic MINOR issues with it. I've checked it all over and feel confident that there will be no serious issues with anything coming off.
* The materials are just right for me. Robust enough but lightweight.
* The design, all the pockets, layout, etc. is good and usable. Could perhaps optimize away a strap or two.
* The hip-belt is really good and comfortable. The pockets are the best I've ever seen on any pack.
* I can carry up to two weeks worth of food and gear with it comfortably. The first couple of days it is FULL.
* The pack scales down well on the trip from 10 kg+ food to zero food carried.
* The carbon fiber stay is silly, unnecessary and the pockets the ends go into are too shallow: one of the ends came out from its pocket once.
* The long straps are a non-issue: just cut them to the length that suits your use.
* Hip-belt: having a total of four webbing ends dangling from the belt is actually a good feature as they are half the length compared to the traditional belt design, so they stay out of the way better.
* The belt being separately adjustable for the top and bottom part is nonsense: it is a single piece of webbing going freely thru the buckle and connecting to triglides at each end. It makes no difference which end you tighten, it will even out at the buckle. However, it would be trivial make it separately adjustable by sewing the webbing to itself around the buckle!
* (edit) Correction to above: tested the hip belt a bit and the separate top vs. down adjustment may actually work if the belt is not allowed to slide up or down and is tightened properly (quite tight) along the natural shape of the hip.
What could be better:
* More of an annoyance, but the vertical compression straps could perhaps be something simpler. Maybe just a dry-bag style rolltop closure. They do, however, make a difference when the pack is extremely loaded by stabilizing the whole thing.
* Aluminum hoop stay instead of the carbon fiber nonsense.
* There's something not quite right with the shoulder strap webbing just below the load-lifters. The webbing tends to sometimes slip off the shoulder straps.
Haven't checked the options out there quite recently, but sometime ago there really wasn't anything out there on the market that I'd consider as a possible replacement for 10+ day self-supported trips.
I've also used Six Moon Designs Swift, which is a fairly nice pack. The hip belt is not very comfortable and the materials are not robust enough for me. Had a near-catastrophic failure with its alu hoop stay pocket digging a small hole into the back panel. The lateral pull from the hip belt caused the hole to expand into a long tear in the back panel, which I was able to mend and resume the journey. I like the simplistic design. A version with stronger materials would be nice.
I also have tried an old Gossamer Gear Gorilla that I got cheap off ebay. Completely rubbish. The design is all wrong and the materials don't feel right. Shoulder straps are angled wrong, which is probably the biggest issue. Velcro loops on the inside of the shoulder straps digs into your neck (why they have pockets in the first place is beyond me). The long vertical compression strap is difficult to use. Does not feel comfortable on my back even with light loads. I assume the current GGG, which is a completely different design, would probably work better for me.
YMMV. I mainly hike 1-3 week long unassisted off-trail trips in the Scandinavian Lapland, so I value robustness and light weight for longer trips. A long thru-hike with a light pack and supply depots every few days is a different game that I know nothing about (yet!).