I'm one of the 50% that sent their gun back. My M&P9 shot consistently low and left (6" low at 15 yards). Out of the box, the front sight was off center to the right. Since I agreed with many posters on this forum that the first three of five reasons a firearm is inaccurate start with "shooter" I spent about 1500 rounds trying to correct the shooter before I sent it back. It came back with the front sight centered and, I believe, a shorter front sight. The low-left was gone. S&W was very quick with the fix.
I should have sent it back much earlier since I didn't shoot low and left with anything else I owned and almost everything else I picked up and fired for the first time. In the 1500 round effort to correct the shooter, I lost confidence in the M&P and began shooting it more erratically. It's taken a while to get back the proper technique for this pistol. In fact, I stopped shooting it as much and shot others that I shoot much better. With some time off the pistol. a change in my grip technique for the pistol it's turning out to be a pretty good shooter. (Now it's 1. Shooter, 2. Shooter, 3. Shooter, 4. ammo, 5. pistol that are the problem).