Pro Mag function yes, rattle yes
I picked up several of the pro mags from cdnn (great outfit to work with). Very happy with function but being raised by a mother of german descent i have a pretty good eye for detail and my eye noticed that factory mags fully loaded will not rattle when shaken. This in not true of the ProMag mags which really rattle quite a bit when loaded in or out of the gun. I think this may be overcome by replacing the follower with a factory follower but have not tried this yet. To me it just seems that the ProMag is just wide enough to allow a little slop.
They feed just fine, maybe better because if you compare a promag to a factory from the back(primer)side at the tops near the feed lips you will find that the factory mag has a wide shallow groove for the reicever to grab the bullet during feeding and the ProMag is deeper.
The rattle is only a flaw if you need/desire your equipment to be quiet. I will buy more ProMags despite the noise. For a daily carry gun I think it would drive me nuts if I was fairly active.