Rating: 5 / 5
In the past, I've read in a number of places (including Jim Wood's excellent article on "Keeping Your Critical Gear Dry") that these sacks have failed the submersion test. This problem was also noted in a 2006 review here on BPL.
In early 2008 I bought a 2 L Sea to Summit Ultra-Sil Dry Sack so I could keep my "essentials" (which include a PLB) on my belt in a waterproof bag when wandering around away from camp (on the trail, of course, it's in my pack). Before using the dry sack, I tested it by turning it inside out, filling it with water and hanging it in my shower for 15 minutes to simulate submersion. There was not a single leak and no "sweating" of water through the fabric as has been reported previously and illustrated by photos in Jim Wood's article.
For several years I've used a trash compactor bag as a pack liner. I fought with it quite a bit--when I pushed an item down into the pack it would pop right out again because of the slippery plastic. I therefore decided to get two more of the Ultra-Sil Dry Sacks, one for my sleeping bag and night clothing (base layer) and one for my insulating outer clothing, instead of using the trash compactor bag. I tested both in the same manner before using them, with no leaks.
Unfortunately I didn't save the weight numbers--I wish I had! I did determine that he weight differential between the Ultra-Sil Dry Sacks and comparably-sized silnylon stuff sacks was the same as the weight of the trash compactor bag. Using the dry sacks instead of the trash compactor bag did not, therefore, involve adding weight.
The dry bags earned their keep last summer when I slipped and fell while crossing a difficult ford. I was completely soaked, my boots were full of water and, when I unpacked my pack, there was at least an inch of water in the bottom of it. The whole pack went into the water. I was too far from the trailhead to hike out in a day, so if my insulating outer clothing and sleeping bag had gotten wet, I'd have been in a bad way (the weather was cool and rainy). Not a drop of water got inside!
I will continue to test these dry sacks each year to make sure they are still waterproof.
It appears that Sea-to-Summit has observed the independent test failures of the past and significantly improved this product. I'm very glad to see it!