I’ve used these Sea to Summit eVent compression sacks the last few times I’ve travelled abroad or while backpacking, and found them to be an essential travelling companion. Their main function is to keep stuff dry while also compressing and organizing the contents of my pack. The Sea to Summit sacks are unique in their use of eVent fabric (a semi-permeable membrane) which allows for greater compression and the formation of a vacuum like seal.
Before compressing the sack with the supplied compression straps, the Sea to Summit bag’s roll-top is sealed. Then, by pulling the straps taut, air is driven out of the one-way breathable eVent membrane that lines the bottom of the bag. As air is pushed out something akin to a weak vacuum is formed (eVent is impermeable to water and semi-permeable to air). I found that even after loosening the straps the contents remain compressed (it will eventually equalize as air seeps back in, but very slowly). This dramatically reduces the amount of space soft compressible items like socks, clothes, sleeping bags, etc. take up.
I currently own two, in small (10 L) and large (20 L), and have been blown away at how much I’ve been able to compress into my pack. They are super light (4.5 oz and 5.9 oz, respectively), and as tough as any other compression sack I’ve tried. While travelling in Bangladesh they kept my moisture sensitive camera gear dry even during downpours, and all my tests at home found them to be 100% waterproof (just be sure to not compress anything with sharp edges). While they are more expensive than traditional dry sacks, the superior compression and vacuum-like seal really make it worthwhile when trying to minimize pack space while maximizing protection.