You'll find the M&P highly regarded here but also respected on other AR sites. That hasn't always been the case as just a few years ago, it was the "newer" entry into a crowded market. Over the years, it's proven to be a solid rifle with most of the parts being made in house. Not many other manufacturers can claim that. It's also been extremely reliable and pretty much problem free. For what you are paying in the M&P X, T or MOE range, there are other options and manufacturers. We all saw and heard the same during our research. However, for me, I wanted the M&P. With all else being equal (and the M&P seemingly more reliable) that's what I went with.
And I'll echo the 15-22 comment. It's great to shoot the 5.56 but blowing through 10 mags of .22 is an economical way to add to the fun. Best of luck!