Thanks for the report. I know a police firearms instructor who swears by the Magpul Glock 17 magazines he regularly carries in his Glock 19.
To supplement my original post, here is a picture of the Magpul 21 rounder sandwiched between an OEM Glock 17 magazine and an OEM 33 round "fun stick," followed by a picture of an OEM Glock 17 magazine in a magazine pouch next to the Magpul 21 rounder. It creates a lot of options in terms of spare ammo for Glocks.