Polymer magazines were first introduced in 1977 with the Steyr AUG rifle but they did not hit the mainstream until 2007 when Magpul introduced the first polymer AR magazine. Like aluminum mags, people did not quite accept this new fad until more understanding of this truly space age material become well known.
There is a reason why polymer is used to hold together the international space station. Polymer is UV light, chemical, and corrosion resistant and won’t get scorching hot after sitting in the sun during a particularly long range day. Initially, the mainstream looked down on polymer assuming it to have plastic characteristics but in reality, it features elastic characteristics meaning it can deform and bounce back into shape. Like aluminum magazines, this has pros and cons when it comes to the magazine feed lips.
Over long periods of time, the pressure of a fully loaded magazine spring can deform the feed lips permanently causing them to not hold the rounds in the proper orientation for reliable feeding. However, Magpul ships dust covers with their magazines for this very reason that will prevent the rounds from applying pressure on the feed lips for long term storage.
Polymers have come a long way since the early days and for regular use, it will be a long time before you see degradation in the performance of your polymer AR-15 magazine.
This next point might not seem like a pro, but the elasticity of the feed lips does have a breaking point. Unlike aluminum that will bend without you being able to immediately notice it, when polymer feed lips fail you will know immediately, and you can ditch that mag from rotation. It is better to be 100 percent sure that a magazine will not work than to install a magazine expecting it to work and causing issues.
Polymer magazines are the biggest subsection of AR-15 magazines on the market today with more and more offerings from other popular brands like Amend2, Mission First Tactical, Troy Industries, and Hexmag. Polymer magazines are right in the sweet spot between aluminum and steel in terms of cost and they all have their own unique exterior patterns.