After trying the Ameda bags (came free with my pump), the Medela bags (bought in a pinch), and these, I have to say, there's no contest, and for one key reason:
they're long and slim!
Duh, it's simple physics. If you throw a bag that's longer and flatter in the freezer, there's more surface area, so it freezes faster. Same goes for thawing-- a bag with more surface area will thaw faster.
It ain't rocket science but I just can't figure out why other manufacturers haven't caught on. The other bags are smaller which seems nice and all, but they're just not as handy when you have a crying hungry baby and a huge block-bag of milk to thaw.
Add to that the double seal closure (not one has leaked on me in more than 24 plane flights!) and you have a winner.