I got one of those and it didn't work (if you're talking about the Korean 'American Tactical' mags). It was bending the rounds as they went into the chamber (57upper, not a PS90).
I think the ones that 57center.com and MGS have are different, and are manufactured by 57center.com and guaranteed to work in their uppers. (see their website). BTW 29.95 is the MSRP so if anyone is a dealer you might be able to get them cheaper/ do a group buy.