I have one, and I have to say that it is very easy to conceal, and I carry it when I am wearing clothes that would otherwise prohibit carry. Living in Florida, I often have to wear clothes that would make carry too obvious, due to printing. This pistol even disappears under a fairly snug, tucked in polo shirt at work, when I carry in an MTAC tuckable holster. It is accurate, reliable, and shoots well.
My wife has one as well, and we customized hers by replacing the trigger with a new Apex trigger and some TFO tritium/fiber optic sights from TruGlo.
The Apex trigger cost us $90, and was easy to install ourselves, even though we have no real gunsmithing experience. That is another plus to the M&P line of pistols: they are easy to work on yourself. The Apex trigger lowered the trigger pull by about two pounds (to about 4 1/2 pounds)
The TFO sights did require us to buy an M&P sight tool for $150, so that may be a deterrent, but those sights are bright and easy to see.
All in all, the Shield is a good buy.