In reply to zoonalink: I don't know about the airlocks, but I have just received an alpkit gourdon and the gourdon is much thicker material, and is "airtight".
The Lomo ones are light though and I'd recommend them as pack organisers/dividers, just not as stuff sacks or floatation aids! They would be fine inside a decent weatherproof rucksack. My first aid pouch is in one (as an extra level of protection) as well as spare batteries, spare head torch etc.
If you want one to use on it's own, or strapped to the outside of something, I'd say that they were no good.
Hope that helps.