I have a gram's +5 on a glock 20 nfml mag, back when I got the +5 it was during the ban and fml glock mags were pretty darn rare.
Never had any trouble with it and I bought it used and already installed on the mag.
I did lots of research before investing money in the extension, pretty expensive for 5 rounds. As far as glock mags go it was highly recomended and well respected by those who shot in competition where the extra capacity was legal.
I just used it for target/plinking and it has never given me any trouble.
From the research I did at the time, several years ago, the cheaper stuff sometimes had trouble while the grams was considered expensive but mostly trouble free.