With Glock magazines, I've shied away from anything that isn't the current design 2-piece factory extension.  I worked with someone in my old department that put the Pierce on his magazines.  One time on a call, he bumped his gun on the vehicle pillar when getting back into the driver's seat.  His rounds went everywhere (it was actually kind of funny).

The original Glock + extensions sometimes had this issue.  The newer 2-piece design (Parts 7151 (indicated with a +) & 7165 (insert) - both required) won't move.  Direct from Glock, they are $5.00 & $2.00 respectively.  It will add two rounds for 9mm & .357 Sig, and 1 for .40 S&W & .45 GAP (although you usually can get 2-rounds in with the .40 once the spring settles).  Also, make sure you use the 11-coil (33601) spring.  The .40 mags should already have it, but just verify it isn't the 10-coil used in 9mm and the original .40 magazines.

I'm guessing most people are fine with the Pierce, I just wouldn't use them for duty use where they could be bumped hard.