When I was expressing and freezing breast milk these were an absolute lifesaver. We only have a very small freezer, but these pack together and squash flat. They very rarely leak, and they hold a whole feed. Like the other reviewer, we used cups for storage too, but finding breastmilk freezer bags meant that we could use the cups for fridge storage.
When I gradually stopped breastfeeding, we still had a ton of these left, and I wasn't sure what to do with them. I then discovered that they're brilliant for storing individual portions of soup and pasta sauce (and anything else vaguely liquidy). I can now make a load of soup and pasta sauce at the weekend and not worry about it taking up all the freezer or going off: it just gets frozen in individual portions, in leak-proof bags, and then my OH can just defrost what he wants for lunch when he gets home. I've just finished the last lot I had and ordered some more, and they'll most likely be on my shopping list for a long time yet!