The KCI Korean mags have gotten some very good and also very bad press. I am not going to dispel any myths or give you an unequivocal account of their failures or successes. I am going to give you my opinion on the KCI mags that I have purchased and used over that last few years.
First of all KCI mags are very cheap. I found them to be less than half the cost of factory Glock magazines sometimes less that a third the price of factory Glock mags. The fit and finish of the KCI mags is very good. I have disasembled, cleaned and lubed all my magazines and found the interior of the mags to be as good or better in some cases than factory Glock mags. The floor plates and followers seem to look exactly the same and I could tell no difference. I have also had only one malfunction with one of my KCI 33 round 9mm mags.
I’m not sure what the difference in the metal or the finish is between Glock factory mags springs and the KCI mags springs, but the KCI springs are a lower quality. I found them to be slightly thinner in diameter and coated in a black finish. The factory Glock mags seemed to have a more robust feel for them, and they I found little to no wear on them, even though I use them more than my KCI mags. The one 33 round KCI 9mm mag that I had the malfunction with had a slightly twisted spring. I am assuming that I must have either tried to overload the mag or done something to it to damage the spring.
All in all I have found the magazines to be within acceptable limits to carry every day. If I had the money I would buy all Glock factory magazines though. I attribute that to just being a stickler for matching things. If you are interested in the mags I highly suggest you buy one or two and give them a try for yourself.