I have never tried their standard capacity 20 or 30 round magazines but I do have one 5 and one 10 round JM Mini 14 magazines that I use for ranch duty. The 5 round magazine is used when I'm on horseback and carrying in a scabbed.
The 10 round gave me feed issues right out of the box but was an easy fix with a pair of pliers, the left side feed lip was a bit low, could of been hit or dropped at the factory and got by QC as the packaging it came in was not damaged when I bought it. After the fix they have both worked 100%
I have about fifteen of them from the ban period. All worked perfectly. They were considered the closest to factory mags. I've since started going with 20-rounders - haven't even looked to see if Masen sells those, as I grabbed a bunch of factory mags while they were available around $20 a few years ago. The 30s are a problem shooting off bench.