There are two types of Promag Glock mags.
The first is all plastic and are total junk. They are very thin and have witness holes in the back. If you see these, you'll know, cause they look REALLY cheap.
The second type are made like the factory Glock mags, with a metal insert inside. These are heavy (like the original Glock mags) and have no witness holes.
I have three of the second type of Promags and they have worked fine. I used two of them for a 300 round class and they never even blinked. For the $30 that these mags usually cost, I think they are a pretty good deal.