I've been experimenting with Mylar bags for tobacco storage for the past three years or so. Mylar, used in a laminate with other plastics and foil, can provide an absolute moisture, oxygen and light barrier. The quality of Mylar-laminate bags however varies considerably. I suspect that some of the cheaper bags don't contain much, if any Mylar. My observations:
1) The thicker the bag, the better. 4 or 5 mil is what you want for long-term storage.
2) The Ziploc seal is the weak link in the chain. Some bags seal well initially, but all bags will leak over time. More so if they are opened closed more than a few times.
3) The cheap ESD, transparent Mylar laminate bags are great for short-term storage, not so good in the long-term. It's the zip seal that fails. I put up some blends two years ago in cheap ESD bags, and opened them last month. Some were still perfect, others bone dry. Bad zip seals.
4) Your best bet is to repackage your baccy into 50gr. amounts, then heat seal the bag. That eliminates long-term zip seal problems.
5) Keep in mind that the FoodSaver-type vacuum bags aren't Mylar laminates.
In short, storing tobacco in Mylar-laminate bags can be a good way to go. You have to be certain of the quality of the bag, though and with the market flooded with cheap Chinese goods, that task becomes difficult.
Sorbent Systems is a good vendor, but their website is difficult to navigate, and their minimums can be high.
I now use zip-seal Mylar bags for short-term use only. I store all my open tins
in these bags, and so far, it's working out great.
I use heat-sealed Mylar laminate vacuum bags for some long-term storage, primarily for large amounts of whole leaf, but still use Ball jars for everything else. I've never had a jar seal fail, and glass proven itself over lo these thousands of years.