Just make sure they are genuine Glock. I have seen other plastic glock mags, I think Korean, that were total crap. I have found the 33 round (27 round in .40) are just as reliable as any other factory mag.
Yep, South Korea is currently exporting clones to the US, and Scherer is making clones in the US. I stick with the real Glock 33 rounders.