Didn't know the S&W lower was now made by Magpul. That's kind of cool.
The M&P is an interesting carbine. That barrel for instance is most likely a Thompson Centre 5R barrel. Anyone who has shot a T/C Icon knows they make barrels that can shoot. The 5R often leads to easier cleaning and a tiny bump in velocity over other rifling. The 1:8 twist is also interesting as it is considered great for target barrels. I know the earlier Stag rifles were 1:9 but actually almost a 1:8 twist. Stag made the M&P15 for the first year.
Interesting choices and I could see it being a very good AR. But it is taking a different path. I wouldn't rule one out and I'm sure it would be decent but... anytime I've done comparative shopping the M&P15 hasn't been at the top of my list. It seems to be trying to be a bit of everything. Light handy service style rifle but target rifle options with the rifling. With the AR specialization is the key. If someone is looking for a plain AR they generally look for a milspec based one with as many of the regular featured found on the service version. Reece/Recon then a more match grade barrel with free float handguard. I'm sensing they are appealing to the newer hobby shooter at this point.
I look forward to seeing what they come up with in the future as they seem to be fairly quick to change things up. They have a good name and make a decent product.