jor5697463 wrote a review of Sea To Summit Ultra-Sil Nano Dry Sack on November 29, 2013
Familiarity: I've put it through the wringer
I use this for two purposes: 1) As a wash bag for clothes when I travel and laundry service/laundromat are not available, 2) As a backpacking/hiking waterproof bag for electronics (phone).
The material feels a bit delicate, as it is see-through thin, and if given enough abuse, I wouldn't be surprised if it ripped or punctured. However, you would need to try in order to damage the bag beyond normal usage.
For all uses so far, it keeps water out, or for my example described above, keeps water in.