- Better reliability, particularly M885A1 ammunition, because of the improved geometry of the follower;
- The feed lips are more resistant to deformation (e.g., bending) than the USGI magazines, and if the magazine is damaged, it will be clearly broken;
- The ability to quickly disassemble the magazine for maintenance and cleaning, whereas the aluminum magazines are more difficult to disassemble, and it is easy to bend the tabs that hold the floor plate in place;
- The windows on the Gen 3 make it easy to estimate remaining ammunition;
- The need to download the GI magazines to 28 rounds to allow seating the magazine with a closed bolt, but that the PMAG allows sufficient compression to allow a full 30 rounds;
- The PMAG is more reliable in the M249 SAW; and,
- The better reliability and geometry of the PMAG allow it to be used as an impromptu monopod, whereas putting pressure on the bottom of the aluminum magazine can induce a malfunction.
Ironically, however, compatibility with different weapon systems may be a reason not to select the PMAG. In the video, below, Alex C. with The Firearms Blog explains why he won't buy PMAGs is because they do not work with other firearms he owns which, nevertheless, use STANAG magazines.
He specifically notes that PMAGs generally won't drop free from most of his other weapon systems that use STANAG magazines, and won't seat in some of them (e.g. the FN 2000); and using a PMAG can actually result in damage to the FN SCAR. He also notes that the standard GI magazine is less expensive and weighs less.
Although not mentioned by either of the foregoing, I would also note that some of the older magazine pouches will not accept PMAGs (at least if you are trying to put two in the pouch and still want to snap the cover closed).
In short, if you are using the AR system, the PMAG is a great option, but if you have other rifles that use STANAG magazines, you may have compatibility issues.