Magpul PMAGS are the best things since Sliced Bread for the AR-15.
They had some issues with the earliest PMAGS, but that issue has been long fixed.
Don't waste your time with HK mags...crap, IMHO.
My gun dealer still thinks that it would be prudent to buy a few aluminum mags, just to be prudent, in case PMAGS really do flop. But that is highly unlikely.
Testing on the PMAGS have included sustained full auto fire in M249 SAWs. You should go to M4Carbine.net and read about the PMAG there. They have also been through far worse. About the only problem I have read about is in exxxtreemmmee cold, they may have problems.
I think another mag to look at would be the Lancer mags...
Stay away from Triple K, Promag, National Magazine, and such.
There are several good brands of aluminum mags, too. Look over at M4Carbine.net. I haven't bought much in the way of aluminum mags, mainly PMAGS, since they do work well.