I would consider it to be quite water resistant and have used it in rain and the contents did not get wet. Of course it could not be considered waterproof it does have a zipper that's not sealed, nor are the seams. The ultra-sil is the same the sea-to-summit ultralight stuff sacks.

I use it on tour for day hikes and store runs and in my panniers to separate clean/dirty clothes. Also to note: the shoulder straps very flimsy being doubled up ultra-sil, I would not consider it equivalent to the (4 times heavier) Flash18.