I consider myself an expert on very few things, but this product is one of them.
Take a look at my profile picture. See the bundle on the back of the bike? I lived out of a large e-Vent sack for 30 days straight. it held a hammock tent, a sleeping bag, and all my clothing.
Nothing ever got wet. Not once. No moisture buildup. The e-Vent bottom even allowed some things to dry inside the bag, which means a slightly damp sleeping bag like a down bag won't sit there and stew all day if it's on the outside of your pack.
It's very durable- I still use it. It's probably seen 60+ nights of use.