Everyone seems to love Magpul P-Mags, but I have had issues with them, at least the 30 round, Gen 2 version. I have had about eight of the Gen 2 , thirty round AR/M4 P-Mags, two with windows and the others without. One of the window mags and three of the non-window Gen 2 mags would fall out of the receiver of my AR under recoil. The other window mag and other three non-window Gen 2 P-mags worked fine. This problem persisted even after I replaced and repeatedly adjusted the magazine catch.
It may be that my receiver is slightly out of spec, but I measured the width of these Gen 2 P-mags with a caliper up where the slide catch lock recess was and found significant variation, even though the mags had seen little use. The wider mags worked fine. The narrower mags fell out.
I have since bought only Gen 3 P-Mags (when it comes to Magpul magazines) and I have had no trouble with them. These seems to have stronger, reinforced side walls and I suspect they flex less during recoil. I have a number of 30 round, 20 round, and 10 round Gen 3 P-Mags and they have all worked fine. I prefer the 20 or 10 round mags for three position shooting. The shorter mags work much better shooting prone. The 10 round magazines allow the support hand to be placed directly beneath the magazine well which is the balance point of the rifle.
In addition to Magpul mags, I have a couple of aluminum GI mags which I have not used much, and several Lancer magazines with metal feed lips. The Lancers are by far my favorite and I would probably use nothing else if they were less expensive.