When I first encountered Mylar vacuum bags I wondered why I would ever use them. Then I started paying attention to all of the foods that I purchase that are sealed in mylar bags instead of plastic and I started to realize there were absolutely foods where metallic was the best option of fresh food.
When combined with an oxygen absorbent, mylar vacuum seal bags are perfect for long term food storage. This allows me to make my emergency storage supplies at a much lower cost and have much tastier food than what can traditionally found on the emergency preparedness market. This ability to store grains, dried fruits, jerky, and other emergency staples for the long term has made it so that our emergency supplies are more diverse and higher quality.
In addition I have started making larger batches of granola, dried fruit, homemade chips, dehydrated potatoes, and jerky for the camping season and putting those into these bags. By making my own instead of purchasing these items from the sporting goods store, we can now go backpacking more often keeping our packs light and our meals enjoyable. Combined with a dehydrator, I even make soups and stews that I dehydrate and vacuum pack to take on the trail.
Outside of camping and emergency prep I have used this type of bag to store herbs, bulk purchases, and also for dehydrating and storing. I have had herbs and spice mixes last for multiple years, staying just as fresh as when I first packaged them. I also do this with nuts, which has allowed me to purchase in bulk, often directly from local growers, and put them in small packages for later use.
In addition to these uses I recently started bagging my own bread and cookie mixes. Some of these mixes I give away as gifts and others stay in my pantry making it so even my kids can easily participate in baking using the mix and the easy instructions I attach to the bags. The economy of purchasing in bulk has decreased our food cost even when the cost of the bags is factored in.
My basic strategy with deciding on a mylar bag or plastic is how it is stored. In the fridge or freezer I use plastic. Goods that will be stored in the pantry or that needs to be stored at room temperature I use the mylar bags.