I don't even own an AR but I have several of the black steel HK mags. I have the polymer HK 416 mags too. If HK ever releases/builds an RAL 8000 MR556 in the US, I'll finally have a rifle to go with the mags I have collected over the years.
I'm an AK guy so the weight of magazines doesn't bother me, of course I don't carry multiple mags on me typically, and when I do it's usually G36 mags as the G36 is my primary in professional application. It's a compromise really, heavier mags, more durability generally. Ever handle a slab side AK magazine? They are made of 1mm thick steel, the original AK magazine, they are beautiful, and built like tanks! Some polymer mags are more resistant and "bounce back" against impacts, but when they break, they break. Steel dings or dents that impede follower movement can sometimes have the damage knocked out, or feed lips tweaked back if they get bent up, but the wiser option would be to replace it if the mag is used in serious or professional application.
Like James said, they are replaceable items, so buy good mags as cheap as you can, when you can, and stack them deep!