Rating: 5 / 5
I have the same configuration as the above reviewer, but with 2 shoulder pouches. I also have the Blast 18 (equivalent in size and design to the Zilch).
These are the only packs I know made of this more durable variety of cuben fiber, which is a lot more durable than silnylon or spinnaker and lighter than nylon of equivalent strength. The material is tough, waterproof (there are not many seams, and you could seam seal them yourself), and attracts a lot of incredulous stares and questions.
This is a tentative "5," because I've used the packs only on a 3-day hike up Pikes Peak, but my companion and I like them so much that we have chosen them above other alternatives to take on our Colorado CDT thru-hike.
The packs have two large, usable side pockets for water bottles, and a large back pocket that is generous enough to not have to think too much about whether or not there's enough room to put things back there. The design is simplistic and ingenious and allows for attaching trekking poles or GoLite umbrella to the side easily.
With the four pockets on front, I have lots of room for my rather large camera (Canon G7) in the shoulder pocket, toiletries and random items I may use on the trail. I have found the padding on the hip pockets very sufficient for loads up to at least 20 lbs, and haven't been above that yet. The pack carries very comfortably for me.
In short, this is the lightest pack of its size with this degree of durability, and has a simple, yet highly functional design with side and back pockets that are larger and more useful than other UL packs I have seen.
Added 6 months and 50 trail days later. The pack has held up well. I would project its lifespan at 100-150 days. It's starting to get frayed in places, but no functionality has been compromised. Pack is comfortable to 12 kg (25 lbs), which is about standard for frameless packs. I continue to like the features of this pack and do not see any significant shortcomings.