These things rock. Huge and strong, I put lots of clothes in them, zip them most of the way up, then suck the air out with a vacuum cleaner, to make my clothes take up half the room they would otherwise. This, along with good folding and creatively packing the sealed bags has let me fit more into my bag than I thought possible.
One trick though: they don’t hold the vacuum forever. I’m guessing they just leak air from the zipper. No big deal; vacuum pack all your stuff and throw it in your bag real fast, closing it all up before they have a chance to expand.
This probably wouldn’t work so well if you’re travelling through hotels and won’t have access to a vacuum cleaner to pack the bags back on leaving, but it’s still a great trick.