A few months back I bought some ETS and Magpul Glock 19 mags to test out and use for practice. The idea was that I can get these for less than factory mags, use them as practice mags and save my factory mags for actual carry. I also wanted to evaluate the ETS and Magpul mags to see if I would ever consider rotating them into actual carry use.
I bought 2 ETS Glock 19 Mags and 2 Magpul Glock 19 mags.
So far, I have about 100 rounds through each magazine, primarily steel cased Wolf 115 grain.
The Magpul mags have had zero malfunctions so far.
The ETS mags have had numerous failures. The issue seems to be that the steel cases are not "slick" enough and they get "wedged" against the side of the mag. This usually happens after firing the first 5 rounds or so. Once it gets stuck, obviously it won't feed. This has happened with both mags multiple times. I realize that 2 mags is a small sample size, but based on what I have seen, I would not recommend the ETS mags.
The Magpul seem fine, but I will continue to use factory Glock mags for my carry. Once I have put a few hundred more rounds through the Magpul mags, I might consider using them for carry.
TL:DR
ETS mags failed consistently with steel cased ammo. Magpul mags have been fine so far.