I have one for my G20. I have put about 1000 rounds of different types of ammo through it now and have had a lot of fun with it.
I get a few failures to feed now and then but in general it functions very well.
I would highly recommend this option if you are a Glock owner.
CCI Stingers, Mini Mags, SGB's, and Velocitors give me about a 1 1/4 " 20 shot group at 10 yards with the SGB's giving the same size group at 20 yards.
Win 37gr HP and 40gr h/v solid ammo works very well as well.
I bought mine on E-Bay.