Not a good reputation for stopping power, but has all sorts of other reputations. The Mujahadeen during the Soviet occupation of Afghanistan called it the poison bullet because wounds from it were incredibly difficult to treat. Although the fact that they're describing people surviving being shot in battle for a number of DAYS afterwards is a little offputting.
What is absolutely not disputable is how affordable and available the ammunition is right now.
I personally already own an M&P15R. I'd say it's a great rifle (which I do believe it is), but a more accurate statement would be that 'it's as good as any other S&W AR'. I can't really tell it apart from any other AR when I'm actually at the range with it.
There is one issue that could be important though, and that's the fact that spare magazines were impossible to find when I first got my M&P15R. I don't know if that's still true, but at that time, literally every single magazine that S&W was getting was getting shipped out with a rifle. C Products started making a 5.45 AR magazine but their reputation for reliability wasn't very good at that time.
I decided to order one of the modular lowers from MGI military and put an AK74 magazine well on it. Problem with that is the lower and the magazine well cost about as much as the M&P15R did in the first place, so that's another thing to consider.