I got my Shield 9mm the first week they were released. I have shot nearly 2,000 round through it with zero failures of any type.
With ANY handgun caliber, it's bullet placement. No amount of caliber is going to make up for poor shot placement. The last FBI statistics I saw, showed only a 3% diferrence in the stopping power of the 9mm, .40, and .45.
I prefer the 9mm to .40 in subcompact pistols such as the Shield. Better recoil control, and a faster follow on shot. And with a good 9mm defensive round in the pistol, it makes it a formitable weapon.