I have used them quite a bit in a G19. I have and use the 15, 17, 31 round versions. I like them and they all work very well with brass cased ammo. If you shoot a lot of steel cased they can get frustrating because the steel case and polymer wall can slow down the spring pushing up just enough to cause malfunctions. This happens to me with magpul glock mags as well but not with OEM mags because of their metal liner. If sticking with brass then I think they are a great option and have the same base plate design as the OEM mags so that is nice as well if wanting to put on magazine extensions.
Worked fine in my G19s with 15, 17, and 22rd versions.
Pretty well in my G42 with 7,9, and 12rd versions, although some of them don't hold the slide back after the last round (spring issue? IDK).
Absolute shit with my G43 in 7,9, and 12rd mags. Work with ETS and they said it was the gun, but it works fine on OEM mags. FMJs worked pretty well with ETS but JHP / Flat nose bullets always nose-dived / got hung up.
Eventually threw all the G43 ETS mags away. G42 and G19 ETS mags, I still have them but only for range work.
OEM glock mags are hard to beat.
Concerning PMAG Glock mags, they work great with brass but meh with steel. I have some of those but like ETS mags, only for range use.
I ran the clear extendos in my NAK9 and FX9. They operated great in both guns. Hard to beat the quality for the price. I kept them loaded for multi month stretches and no flexing or issues with the mags.
I have used them quite a bit (approximately 5,000-7,000 rounds of competition) with my Glock 19. I noticed they dropped freely very well when they are new. Over time, the slick plastic got ate up and developed enough friction to hang up on dropping. No feed malfunctions at all. I noticed similar results on the Glock 17 as well. They have been shifted into my dry fire mags and I ended up moving to Magpul mags for live fire and haven’t looked back. I’m not sure if Magpul has started making Glock 48 mags yet.
If you baby them, probably no issues. Make sure to wipe them off well if you drop them a lot in the dirt.