I have a bout 20 of the Korean 17 mags, and shoot them every week in practice. They have been fine, and seem to be just as reliable as the factory mags. Priced at basically 2 for 1 (early on, it was 3 or 4-1), its hard to beat the price.
After a few years of constant use, I did replace the springs in 10 of them with Wolff springs. Dont know exactly how many cycles they had when I swapped them out, but I shoot on average, 300+ rounds a week, so that should give an idea. I cant say the factory mags would be any better with all that use, as I dont have a baseline with them to know.
I also have both Korean and factory 33 rounders, and both have had issues. I found two of the Korean mags in my safe, that had been loaded for a couple of years and forgotten about. They didnt fare well at the range the next time out. Lots of malfunctions. The other four I have, that werent loaded, seemed to be fine, and worked as they should. I replaced the springs and followers with Glock parts in the two that had problems, and they seem to be OK now.
The Glock factory 33 rounders havent been all that great either. They have had issues too, although they seem to be getting better with use. I dont trust them for anything serious either.
I wouldnt have a problem using the Korean 17 mags for anything serious if I needed them. As far as Ive seen, they seem to work as well as the factory 17 mags I have. They are great for practice mags, and worth it just for that.