Haven't had the opportunity/unluckiness to get caught in a heavy downpour. But was in soft snowfall in Hokkaido for 30min or so and the Fastpack held up fine.
Now after adding a 70D body and an additional flash and extender, I feel that I have outgrown the Fastpack, hence I bought a F-Stop Loka bag instead. Amazing quality, 20year warranty, light aluminum frame that is removable, can swap out the ICUs if I expand or shrink my gear, depending on the location and also with space for a 13" laptop and clothes. Can add a hydration pack, rain cover, etc. It is the perfect backpack for me.
I can remove the ICU and it becomes a full fledged backpack or place either the small or medium ICU (I bought both) depending on the gear. My Loka can fit both small and medium ICUs, if needed. The bag can still fit in the overhead compartment of a plane or bus and if I am forced to check in the bag, I can remove the ICU quickly and handcarry the ICU with me instead. Looks understated and does not scream CAMERA!
Downside to the F-Stop? It is expensive! S$388 for the Loka and about S$100 per ICU, depending on size. Rain cover is optional and gatekeepers are S$28/pc.
But after saying all that, it is, to me, the ultimate backpack. Now's the best time to get one because F-Stops are having a 10% discount.
Not a salesperson, but just wanting to share the best camera backpack I have seen or researched.
Cheerio!