There are two ppl in my company that have spent money on the H&K mags. Both of them say that these mags are the best things since sliced bread, pizza, and other such tasty things. I've been using USGI stuff since I signed up (save the purchase of a couple of 20rndr's from Brownell's) and have had no problems with them at all. Of course, I'm smart and don't bang my mags against my helmet to "seat" my rounds, as that messes up the weld on the back side of the mag.
Do I think these $50+ mags are worth it? Not really. But then again, I've not used them when it really counts. Are they sexy? Hell yes. If you really want to know, buy one (1) and put it through all sorts of hell during a year's worth of range time. If you think you got fifty bucks worth, then you got what you paid for.