I love this backpack, in fact I already own his 20liters version (that I largely reviewed on https://riccardoparenti.com/project/arcteryx-alpha20-review).
The Alpha AR 35 is a pack designed with alpinism in mind. The the N315r LCP, this ArcTeryx patented material feels very durable and reliable. It didn't show any wear after a year of intense use on the rocks and trails. Regarding waterproofing, I would say no pack can be considered full waterproof (except diving bags). Still, the Alpha AR 35 repelled the heavy rain of a Japanese Typhoon beautifully during the rain season. So far, my computer and all my belongings were safe; I guess the test passed!
This backpack is engineered for mountain activities; The capacity allowed me to fit my alpine kit plus light bivouac gear. I loaded it to the maximum and didn't feel any problem in comfort. I would say that can carry almost the same amount of gear of my ArcTeryx Alpha FL 40L (new model).
I definitely recommend to people looking for a technical backpack for alpine ascends or simply trekking. I would say it could be overkill for day-to-day use in urban environment.