I have a few because they were $4 or $5 at CDNN not too long ago. They don't like to go in min very well (I think maybe the way my magwell cut?) but they do go in and fire without problems after a little forcing. The plastic starts to press-fit into the magwell the more I use them.
I use master molders in my AR and my Tavor without any problems, I'm actually a big fan of them. They are really cheap from CDNN and work very well. I've never had a mag-related problem using them. Canadian army uses them I believe.
I ordered a few from CDNN and they have worked fairly well once I can get them seated into the mag well. The Bulgarian steel ones on CDNN work the best.
I believe they are 45's or something like that? I've heard they were designed to hold 30 rounds nearly indefinitely without damaging the spring but could be topped off for immediate use.
I'm not a fan, for not much more you can get the Polish surplus polymer 30 rounders, or 30 and 40 round polymer mags from Bulgaria for about 10 bucks from Wideners that have metal reinforcements in the feed lips and sometimes in the front and rear locking lugs. I guess they decided halfway through production that the lug reinforcements weren't necessary, since I've literally done push-ups holding myself up off the ground with the mag in the gun supporting all my weight and the un-supported lugs were solid. Didn't try the supported lugs.
I had originally thought that none of them had lug supports, since the magnet did not detect them like it did with the metal in the feed lips. Come to find out the metal in the lugs is aluminum, and not magnetic, and you have to shave off a tiny bit of polymer to reveal the metal.
Tapcos have worked great for me, never had a problem except for my mini 14 mags wearing out the front locking hole, but they feed great, and with some JB Weld around the hole every 2-300 mag insertions, they're still trucking along after all this time. The gen 2 with the metal reinforcements work great in my older minis, but require the locking hole in the front to be drilled out about a millimeter wider than they come from the factory in order to work in newer 580 series mini-14s.
Master molder and Thermold I believe use the same plastic or polymer, I have had cracking issues with both types of mags, and they tend to get soft and extremely flexible and non-rigid when the temperature heats up- not even left in a hot car, just being used on a hot day at the range, loaded mags will spit out rounds through the feed lips, and cracks tend to develop on the spine of the magazines, perhaps on the fault where the two halves were welded together?
Anyways, there's a world of difference between good polymer and bad. All the ones I've listed except for the master molder and thermold have been extremely rugged and reliable.