Odds are any malfunction will be mag-related, so ALWAYS carry a spare. This keeps magazine oriented in your pocket so it's easy to grab. Magazine fits securely but is also easy to remove. Prints like a thin wallet and doesn't "clunk" like a bare magazine does - easier to transfer from one coat pocket to another and protects it from sweat and rubbing against other items.
Pros: Keeps pocket lint & dust out of magazine
Cons: I should have purchased two :-)
This review was written in the old system and had content requirements that are different than reviews written today.