ive used many of them- i also have hex's in card stock- the mylar ones hold up longer- but they are on the expensive side-
and you still have to be careful ironing over them- yes they are (heat resistant) but they will melt if a hot iron is actually placed right on them- you can press your fabric around the edges of them safely-
the card stock ones you get 100 or more for the same amount (from PaperPieces.com) they work as well -but after awhile the corners loose their crispness- and they do wear out-
so it is a choice to make- i have both- and use both pretty much equally- the mylar stays crisp- does not bend out of shape when you are taking them out