Ok.... Ordered a pair of them myself. When they come in, I'll have an extra excuse to go to the range! 
I posted a brief report on the Tactical Rifle Thread, but my G21, with these extended mags, was,......a G21. It went bang every time.
No FTF, FTE, at all. The ONLY thing is when you get to 18-19 rds....you better bring your "man-hands"....They are a bear to load more than 20 rds.
I guess if left with rds. in it for a while, it might help, or just cycling as many as I can get in there. These mags are built tough, and worked as promised.