I am going to be the voice of dissent here, but only a bit. A buddy has a number of KCI mags for his G17. So far they have been trouble free and have run 100%, but he doesn't shoot it that much and may have gotten lucky to this point. I just bought some G19 mags to try and can say they are definitely lower quality than OEM Glock mags. I haven't had time to try them yet, but it is clear you do get what you pay for in this case even if they run fine for me.
Mark