Great for backpacking!
I use these for everything! I recently went backpacking and everything went into a Sea to Summit dry sack. I used the 13L for my tent (Kelty Salida 2) and 8L for my sleeping bag, although after compressing it down in the bag, my tent ended up smaller than my sleeping bag! I've used various other sizes for other gear, clothes, and food. These are very thin (lightweight) but seem to be pretty durable and definitely kept everything dry during several days rain.