No problems for me...
Between us, my wife I have three M&P9's used for instruction classes and range shooting. Two of them have had several thousand rounds each through them. We have had zero problems of any kind. They just work.
My wife handled and shot Glocks, XD's, M&P's... and chose an M&P9 JG. I sent it to the S&W Performance Center for their M&P Action Package which included a new sear and some tweaking. She is very happy with it.
I liked her pistol enough that I bought an M&P9 Pro Series pistol and, being disappointed with it initially, had Apex do their Level-2 trigger job on it. REALLY nice!
Out of the box, the trigger is certainly serviceable and a good argument can be made that you shouldn't mess with it, especially if it is a defensive weapon. In our case, the M&P's are range toys only.
If you like the M&P, I don't think you will regret going in that direction.