Three reasons why I love the Pmag.
1.) Anti-tilt followers. Also tactical 'spank' to get the mag to seat on a closed bolt isn't really necessary (Contrast with USGI with stock followers and Lancer L5s. Those need to be slapped hard to ensure a firm seat.) Magpul's antil tilt followers are a little shorter to allow easier seating.
2.) Feed lip cover. Can store magazines loaded with out fear of feed lip deformation, whether real or imagined.
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3.) To Hell & Back reliability. Seen demos of one feeding and firing fine after a 7.62 round went through it. Another had a broken feedlip and still fired.
I also love the lip on the bottom (gives me a firm surface to grasp when pulling mags from my vest). I love the Maglevel (quick round inspection).
They're basically the most advanced magazines on the market. And with magazines frequently being the #1 weak point on an AR, why not go for the best?
That being said, all my Lancers and USGI mags have been 100%. Just preference.