I don't think that I have ever read anything good about a Thermold magazine.
O'course, most or all of the things that I have read concerned the m16 mags from years ago.
Perhaps the Issue(s?) have been corrected?
That said, if I saw some Thermold M14 mags advertised at, say, ~$6-$8/ea and I already had an order to which I could easily add them, I would probably buy a couple to try. Otherwise, not.