From someone who was
not issued any mags (I'm a civilian
) I've recently have gained trust of GI mags over those highly-acclaimed Pmags.
I've run both GI mags and Pmags in my AR's and they both work well. But for some reason a few of the Pmags do not always lock the bolt back after the last rounds. And these are new/barely used Pmags. GI mags have always locked the bolt back, and I've never experienced any issues with them jamming or falling out of the magwell during 3-Gun or just range shooting. I was a die-hard Pmag guy in the beginning, but the excellent performance of the GI mags I have acquired have made me lean towards them more.
Nothing wrong with Pmags because they are great, just boils down to what works for you and what you can afford.
I'd agree that $15 is a bit much for a GI mag, but that is much better than $50 that some stores are selling them for used! If you had some extra money laying around, I would pick up a few of those GI mags. Extra mags are always good, and you can never have too many.