The stock parts on the M&P are "good enough" for "combat" type shooting and for reliability, but a lot of shooters want to make it feel better when shooting or make it more accurate with their shooting style (me included). Smith & Wesson has made little improvements that have gotten rid of a lot of earlier complaints like gritty trigger (slightly smoother striker block, trigger bar and machining of slide where the striker block rides), slide lock snapping off and lack of "felt reset" (new style slide lock), "sear flutter" and dead trigger (new sear block and spring) and 9mm accuracy (redesigned 9mm barrel). Not all earlier models had these problems and not all needed fixes, but companies like Apex capitalized on these little things to fix and improve the small deficiencies to an otherwise great gun.
For instance, the Ultimate Striker Block (USB) has a rounded dome for the trigger bar engagement rather than a multi-step dome. This gets rid of the "grittiness" of the trigger pull. Their hard sear has slightly changed angles to reduce the trigger pull weight while their forward set sear (FSS) causes the trigger bar loop to engage the sear much earlier to reduce the take up of the trigger so that it breaks farther towards the front. In conjunction with the FSS trigger, the overtravel is reduced which helps eliminate the "jump" of the gun when the trigger breaks.
I think one of the biggest cause of inaccuracy for me prior to doing the Apex FSS kit upgrade was that I was not pulling the trigger perfectly straight back when shooting the gun. My other handguns were an SA 1911 and a revolver. Those guns broke with no pre-travel and no overtravel and were all metal with zero wiggle in the trigger. The M&P has quite a bit of sideways slack in the trigger and pulling the trigger straight back is important for accuracy.
So can you use the M&P with full satisfaction right out of the box for a long time? Yes, I did for over three years. Can you make it better without spending more for the upgrades than the gun (read 10/22)? Yes. Will a $40 part or $125 kit make it a little or a lot better? Some say no, but most say yes.