Looked at some of the other reviews and decided to correct the record.
This product deserves a better rating than it has, based on my experience. I shopped around quite a bit for mylar and this was my first choice considering price, speed of delivery and varying bag sizes.
These allow ZERO light penetration, and seal VERY well, when done PROPERLY.
The bags are thicker than most, and seem to use a higher percentage of aluminum layering.
Very robust construction. Plus the closed end has tear notches for opening after the fact.
Other reviewers should have done their homework and applied correct techniques to ensure positive results, but clearly didnt.
I use a FoodSaver FFS005-033, and employ the hair straightener in tandem with little cut off strips of the bumpy side foodsaver bag material to allow air to be pulled out. This can be seen on youtube. ( not my vid).
If you are expecting your mylar to seal with a stock sealer and get a complete lasting seal, you are using inferior mylar.
The machine isn't designed with that in mind so it stands to reason you will have to do more to get the job done. It comes with the territory.
That being said, I use the large ones for baking ingredients and things like pasta, rice, and oats. The small ones for spices, skim milk powder, and coffee.
I would give 5 stars but the two smaller size of bags are not all that different in size. The medium hold about 5 Tbsp of dry product well, and the smallest about 2 1/2 to 3.
They do seal, they are not horrible, and are not a rip off.
Bought again today.