The reason that I am writing this review, is that so far, this backpack has not received the recognition it deserves as a an ultralight multiday pack. The pack is marketed correctly and carefully, it is a "day-trekking" pack, so as to avoid confusion. If you are not an experienced backpacker, thru-hiker, or "trekker," then this is a day pack. If you are, and have a reasonably light base weight (15-20 lbs.) I would suggest looking at this pack.
First, the pack weighs 2lbs 12oz, which, is right in line with the Granite Gear Vapor at 2lbs 5oz.
After finishing the Colorado Trail this past season with the discontinued NAOS 55, I felt my pack was too big at 55-60 L and 5lbs, I bought the AXIOS 35. The pack is meant to hold between 20,40L. It can do this. For a two day test trip, I carried a bivy, sleeping bag/pad, one set of clothes, down jacket, rain gear, misc. small stuff, and cold food. I had more than enough room. I'd say I could have carried a stove, hot food! with up to three more days of food.
The pack looks questionably waterproof. It is definitely not waterproof. After ten minuets of rain it needs a pack cover or liner.
There have been complaints on the suspension system, the shoulder straps being too close. This for me was not a problem. If the pack is adjusted appropriately the fit is comfortable. The hip belt is great.
The details: Arc' Teryx put minute attention into the design of this pack in order to keep it as light as possible. The buckles, clips, straps, and zippers, are smaller than any I have ever seen on a pack. The kangaroo pouch is incredible. It allowed me to carry my wet bivy, or wet rain fly, on the outside of my pack. Everything inside continued to stay dry. The brain. Two zippers! hate digging around for water treatment in a huge one zipper brain with clothes, hats, or other bulky items in there? No longer; bulky goes in the front zipper, small items in the back. Easy. The hip belt which is quite comfortable has very secure stow pockets on it too. To you more dedicated ultra-lighters, the brain is removable, and a sleeping mat can certainly fit buckled atop the pack. There is even a side access zipper. Luxury. I find it bit a unnecessary for such a small pack. But it's there, and still the pack weighs just over 2lbs!
So for those of you with UL solo tents, 30 degree down bags, and 3/4 length sleeping pads, consider this pack your standard go to for overnight trips.