IMO the STANAG magazine is the weakest link in the AR platform, therefore purchasing good (not necessarily exorbitantly priced premium) magazines is a smart move for longevity and reliable service. In my quest for good quality examples I have tried many brands and types of AR magazines, including the Thermolds; the following is a brief synopsis of my findings:
Good: USGI (cheap and easy to find, seldom used but still quite good)
Better: Magpul PMag (great general purpose magazines)
Best: Lancer L-5 (used for HD/SD)
The above three have proven to work well for me (and are currently the only ones I use) despite moderate cost (none are particularly costly). OTOH many others, while slightly cheaper, afford inferior durability and/or reliability. The Thermold ones, while not horrible, didn't perform spectacularly well either (there were a few malfunctions, but I don't recall ever having broken any), about on the same level as C-Products magazines IME. At the other end of the spectrum, there are Pro-Mags and nameless/counterfeit Al. magazines, these have been horrible and no matter the cost should be avoided (but if tempted, go with the Al. ones...their scrap value is higher).
WRT followers, the SS CMMG followers have worked best for me, but require some modification (to the magazine and/or follower) to fit in some magazines (like L-5s and 20rnd USGI). The Magpul followers are not much worse (just not as smooth IME) and require no modification to fit most magazines (and are polymer so trimming, if necessary, is a great deal easier). Additionally, all magazines (despite capacity or profile) provide better service when "anti-tilt followers" are installed, because even the straight 20s slightly tilt the follower due to the taper of the cartridges. This can, and has, caused stoppages and jams. I don't believe that leaving them out is worth the risk or aggravation.
FWIW, I prefer the 20 round variety much better because they are more compact (particularly useful when firing prone or from a bench), lighter, and ammunition generally comes in these convenient little 20 round boxes. If you need more ammo then you probably aren't doing something right, or haven't enough friends on your side of the argument.