Speaking from experience, if the mags aren't Glock brand they will probably be of very low quality. I had some 17rd USA brand mags, they were all metal, with a cheap spring, and a crummy piece of messed up plastic called a follower. Upon intial range testing the mags worked fine, except they would not hold the slide back on empty. I figured this wasn't anything to worry about since they fed fine. So I loaded one up with golddots and threw it in my safe. One night there was an armed robbery about a block from my house. The guy was on the loose and the cops were looking for him. I grabbed my Glock to load it up, just in case, and the freaking mag wouldn't let me chamber a round. They don't fit tight in the magwell so the actuall magazine body was holding the slide back. Keep in mind that I had fired 200 rounds through the 2 USA mags I had, and never had a problem, until I needed them. They were given away when I traded my 17 for a 27, I told the guy they should only be used for range work, so that the same thing wouldn't happen to him. Some will say that the metal mags will chew up the mag release, I never had this problem, but they only had about 100 rounds through each of them.
In short, if it don't say Glock, Inc. on it, don't waste your time, unless you just want them for plinking.