Thermolds are fine range magazines and in that role they can be useful.
However in a SHTF situation that is the least bit dusty or sandy these mags become non-functional with predictable regularity.
(for instance toss your AR w/ thermold into the bed of your pick-up truck or strapped to the back of your 4-wheeler and run down a dirty dusty road till rifle is covered in dust, you'll have a problem with them almost immediantly as they don't perform well in such conditions)
They work for the most part fine, except in extreme conditions and are best at saving wearing and tear on your USGI magazines.
FWIW, seems they function extremely well in RRA rifles/lowers, don't know why but my RRA likes them even more so than the Bushy or Colts, but I would never depend on them in the conditions described.
They are frequently for sell on the Equipment Exchange.
Mike