I wouldn't worry about trusting it as long as the barrel or extractor has been replaced when you get it back. If you have any malfunctions send it back again. I have over 2000 rounds through mine without a single malfunction. It is a carry gun and you need to be able to count on it. It isn't you, it isn't the ammo and it isn't the magazine. Something is wrong with it, and I would continue to send it back to S&W until you have 200 or so failure free rounds through it. Then start over like its a brand new gun and trust it when it gets to whatever round count you would normally want out of a carry gun until you feel good about it.
Crappy luck, but like it has been mentioned, it's bound to happen with a massed produced gun.
Crappy luck, but like it has been mentioned, it's bound to happen with a massed produced gun.