I want to let BPL'ers know about the dry sacks from Mountain Equipment Co-op (MEC.ca). I think these are new. MEC is sort of the Canadian equivilent of REI.
I have the 7L size which I easily fits my Ultra 20 quilt and then at night I fill it with stuff for use as a pillow. This dry sack weighs 41g (1.4 oz) after I trimmed the pocket off the bottom. These dry sacks have a velco pocket that sorta hangs off the bottom that allows you to stuff the stuff sack into. The pocket adds about 10g (0.3 oz).
Without the pocket, these sacks are just as light as the lightest sack in this comparison test although I guess mine is a bit smaller than the sizes used in the test.
The cool thing about these dry sacks is the price. Mine was just $8 Cdn which is about $7 in USA funds. They are available in 5 sizes which go up to 28L for $15.
I filled mine up with water and held it for a while and no water was leaking out the seams. The roll top isn't as ridgid as some drysacks, so you want at least two rolls in...probably 3....to get a good seal. I'm not sure how the roll top would fair in BPLs test, but the body of the sack seems to be watertight. For my uses (keeping my down bag dry inside my pack) this sack is definitely waterproof enough.
Hopefully this doesn't sound like an ad. Previously I was using the 10L SealLine storm sack (the same one used in this test) which worked well but it was a lot heavier (40g vs. 77g) and I didn't really need 10L. Now my dry sack is half the weight....plus the color matches my Ultra 20.
MEC also makes similar dry sacks in larger sizes that are intended for use as a pack liner. These are $11 - $18.