There's no such thing as a waterproof pack cover, so there's no reason to expect that this one will keep your pack fully dry in a downpour. But it does keep the pack dry in light, short rains, and merely damp when the rain is heavier or longer.
But the major problem with this pack cover is that there is no drainage in the bottom of the pack cover, which means that the water just pools in the bottom of the pack cover, soaking whatever is at the bottom of your pack, and adding weight on your back.
Not recommended. Look for one that has a small hole in the bottom of the pack cover to release any water that finds its way inside.