What JaPes said: S&W has made some changes in their trigger in terms of feel and reset.
I did not like my trigger as received it. It was gritty with a less than ideal break and reset. I installed the Apex duty trigger kit in it and now I love the trigger.
So it would be best too get the newer trigger set-up but I don't know for sure how you would know that before buying.
The Apex kit is not expensive if you need to go that route.
WvDave speaks the truth about the trigger. I've read too many reports of the M&P trigger being gritty. When I bought my 9fs, I declined to accept the first and second pistols the clerk brought from the back. They had that gritty feel to the trigger. Third one out was good, no gritty feeling.
The easiest way to tell if the M&P Pistol you have in front of you is of new production with the Shield trigger is to feel the trigger reset. Dry fire the pistol the dealer brings out for you to inspect.
If it has the newer trigger, You will definitely feel the trigger reset, there is no ambiguity in the feeling of it. You will also hear a *click* at the reset. It's hard to explain, but you'll know it when you feel it. Also, take a look at the test fire date on the little envelope with the spent case. I'm assuming that the closer that date is to the present day, the higher the chance that it will have the newer trigger.
The older trigger, you won't feel that reset. You'll let the trigger travel all the way forward and not feel it. It took me a hundred rounds to attune myself to the very light & vague feeling reset point.
...I don't know how to describe it... the Shield trigger reset is like someone talking in a loud voice... the old M&P trigger reset is like trying to listen to someone someone barely whispering while in a loud room.