Is it fully waterproof?

Yes. It has a waterproof roll-top closure and waterproof fabric, so it works as a dry bag as well as a compression sack.

How does the air-permeable base work?

Air escapes through the base as you roll the top down, so you can push out trapped air before cinching the straps. That lets it compress more than a sealed dry bag.

What can I fit in the 5L size?

A sleeping bag, a puffy jacket plus layers, or a small tent. It suits soft gear you want to shrink, not hard or sharp items.

How is it different from a standard roll-top dry bag?

A standard dry bag only keeps water out. This one also compresses its contents with straps, so bulky soft gear takes less room in your pack.

Is the fabric durable enough for rough use?

It is light nylon built for weight savings. It handles normal backpacking use, but it is less abrasion-resistant than heavier dry bags, so keep it away from sharp edges.