I own an M&P9, M&P9c, and an M&P40.
With that being said, I have held off on buying the M&P45 for several different reasons:
1.) All models I have looked at look used.
I know this sounds rediculous, given the whole "a gun is a tool" thing, but by golly I like my tools to look new. All of the 45s I've looked at had a tarnished looking extractor. I didn't care for that.
2.) Magazine capacity
There are other options available in a polymer 45 that have a higher magazine capacity. If I wanted a low-capacity 45, I'd carry a lightweight 1911.
3.) Price
I think the M&P45 is a little over priced right now. Perhaps they are trying to make up for selling some of their other M&Ps for dirt cheap ($399, as of recently in my area).
4.) Grip size
The grip simply didn't feel as good as the rest of my M&Ps. This being said, I did not have the opportunity to experiment with the other inserts.
As of right now, if I was forced to buy a polymer 45 in this class (exluding HK), I would go with the XD, with a tip of the hat to capacity.
I agree with gudel that the grip safety is a gimmick.