I've had a poor experience with the Shield mags. I pre-ordered 3 mags for my G48 when they were first announced along with their aluminum mag catch. I was getting repeated FTF and nosedives from all 3 magazines. I tried their extra power springs, but that didn't solve the problems. I never could figure out what the issue was, so I ended up giving the mags away to someone who wanted to try them.
If I only had problems with one mag I would have attributed it to getting a lemon, but all 3 having problems made me think something is inherantly wrong with the design. Even when they did work correctly, the feeding was never as "smooth" as with OEM mags. Maybe some individual guns just don't like them. That might explain why I had a ton of problems from the start while others seem to have no issues over thousands of rounds.