A natural tendency when one acquires a new piece of gear is to share one's excitement, perhaps via blog. I try to wait until I have enough experience with a piece of equipment before I do an actual review, and an appropriate amount of time has passed to render an opinion on the ZPacks Arc Slim pack. I've put a few hundred miles on the pack, including a full day of drenching rains. In a nutshell, not too shabby.
Anyone reading this probably knows the ZPacks Arc series is basically an updated version of an external frame pack, utilizing carbon fiber and burly mesh. One needs to know my usage to put things in perspective before making an informed decision as to whether this pack will work for you. I wanted the Slim version for off trail days where I may have the pack on for upwards of 20 hours, or multiday trips with a light load, a usual occasion for me. I'll forego the detailed description of the pack as you can read about it on the ZPacks website. I also knew I would be doing some occasional trotting (not quite running, think 5 MPH) on the flatter or downhill segments of my trips. I did not opt for the external water bottle pockets as I planned on using an internal bladder for day trips and wanted to keep the profile clean. I also ordered this pack along with a custom front pack that includes two water bottle pockets if I needed them (I also use two of ZPacks older version hip belt pockets). In another nod to off trail, I had Joe make me a custom front pocket, with the top and bottom sections of stretchable mesh but the central area cuben hybrid; this idea comes from the snaginess of a large mesh pocket when crashing brush. I get the benefits of a large external pocket, the stretchiness and visibility of mesh but with less area to snag. I reiterate that this semi custom setup is made for moving, with items needed during the day available in the front pack and hipbelt pockets, negating the need to stop and take off the pack every time I needed something. Indeed, this system worked well and some days I would go multiple hours without taking off the pack. Understanding that my loads never exceeded @20 pounds, the system works.
• Comfort: Let's start with the frame system, which is designed not only to support the pack but to promote airflow between the pack and the back, reducing the sweat factor. It works really well as a support system….I never had to take off the pack or readjust due to discomfort, even after hours of hiking, trotting, or off trail squirming. To me this is the number 1 reason for endorsement of this system. When a pack virtually disappears on your back, when it becomes unnoticeable and just part of you, then it's a winner. I want to reiterate that I have never loaded this pack down beyond 20 pounds so I cannot attest to the efficacy of heavier loads. Buyer beware.
Now to the airflow: it is indeed much less sweaty, and the possible disadvantage of the center of gravity of the load being further away from your back was unnoticeable in my use. If you tension the frame system more than I do, placing the load a few inches further away, and carry heavy items while executing climbing moves then this may not be the case for you. That's why one's usage is paramount when buying any piece of equipment, and why I stress WHAT I do with the pack so much. I can't say my back wasn't the least bit sweaty, as even the porous mesh against your clothing is still a barrier for moving moisture, but it was a definite improvement over a pack that lies directly against your back.
• Water Resistance: The pack is made of waterproof cuben hybrid and fully seam taped, making it extremely water resistant. During my all day drenching hike, I ended up with a few teaspoons of water in the bottom. This may have been from one stretch where I failed to secure the roll top, but in any event the material does not absorb water, so removing it from the pack was easy with a paper towel. I would always recommend that at least your bottom layer in a pack is in a waterproof stuff sack. I compare this to a different pack I used for a weeklong hike that included rain, mist and hail. That material absorbed water and my first aid/foot kit in a non waterproof stuff sack got soaked, rendering most items inside unusable…..something I found out on a later hike when I needed to address a blister. Take normal precautions. In sum, this pack is about as water resistant as one can get, but it's not a dry bag suitable for dunking. Know your equipment.
• Durability: Beyond this pack I have had good experience with cuben hybrid material. I am still using an older front pack (custom ZPacks) that has seen thousands of zips/unzips, weather, brush abuse and snot bombs. It's not as pretty as it was but it is still fully functional and to this day I carry my camera equipment in it if I don't need my larger one. On this Arc Slim pack the bottom has some minor stains from ground gunk and the roll top exhibits a little dirty hand use, but the pack cleans up well after use and there are absolutely no wear problems from rock, ground contact, or pointy tree limb/prickly devils club/annoying brush encounters. If anything the material seems to make things slide across it easier compared to other fabrics due to a "slippery" hand.
• Overall Impressions: Even this slim version of the Blast pack has plenty of room for practically any multiday hike I would do, but for my long day hikes/scrambles it is more than cavernous to carry my clothing, food, stand by overnight gear and extra stuff for my daughter if she's taking a small day pack. It's as comfortable as a pack can be, at least for loads below 20 pounds. It's practically waterproof and stands up to normal wear and tear. In my case the white cuben is luminous inside making it easy to find stuff with this top loader (and the white keeps items inside cooler in blazing sun). It's obviously customizable. I can run/trot without undue bouncing. If anything goes awry with the pack, I know that ZPacks will fix it in a hurry, probably for no charge. The Arc system works. I'm trying hard but really can't think of many downsides to owning this pack. If this review sounds too good, then perhaps the pack is, too.