The main reason I invested in plastic storage bottles for my frozen milk storage is because I could not imagine using all those disposable bags. I reasoned that the bottles were the most eco friendly choice since they are resuseable and when I am finished I could pass them on to someone else to use for their baby.
Since the bags are so popular and I was curious I did manage to locate what appears to be the only biodegradable breastmilk storage bags available. I requested samples and not only are they leak proof, they are just as easy to use as the other bags available. I froze my milk flat in the bag and stacked them and have a mixture of these bags and the regular storage bottles. No problems whatsoever in defrosting with these either.