OP, if you own a full size MP9, then you are familiar with the system. The Shield gives you a much smaller version. While this sounds like the answer from a simpleton, think about that for a minute.
I got my Shield about 4 days after it was anounced last year at the NRA show. I've had an issued FS for almost 7 years, and a MP9c for a few years.
The Shield shoots as well as my 9c. As a matter of fact, after a few minutes shooting the Shield, one tends to forget that you are shooting a sub-compact and start to compare it to a duty size gun.
The 9c is a good gun, but it's chunky. It's not much smaller than the full size. The Shield best compares to the Walther PPS. I owned a PPS when I bought my Shield. Shot both together during a couple range trips. Sold the Walther. It was a good gun but I liked the Shield better.
I can pocket carry the Shield in docker type shorts. I can pocket carry my Kahr PM9 a bit better. Same can be said for my old J frame 642. But when it comes to shooting these guns, I can shoot the Shield to a much higher level when it comes to putting rounds on target.
One must always remember why you are carrying a gun to begin with. If you want shear comfort, get a Beretta .25acp. Jetfire. I don't want comfortable. I want comforting. Something I know I can carry discreetly, yet have direct access to. Something I can put rounds on target with while under stress. And I want those rounds to be of an effective caliber so that they can stop the threat. (No 380acp for me, thank you)
I took my Shield with me to Sanibel Florida last December. Was on vacation/getting married. Low threat environment, but I am confident in the "shoot ability" of this gun that I wasn't concerned about it being a primary gun. Something I could not say about my Kahr PM9 or my 642.
I am stopping for coffee as I type this, am at work. My Shield is attached to my vest as I speak. It's my BUG while working. I don't even notice it, have to give my side a quick pat to ensure it's there.
My new wife is short with tiny hands. (She uses the small grips on a 9c.) The Shield is a great size CCW gun for her.
I am old enough to remember when the S&W 3913 took the market by storm. Because it combined duty gun performance in a smaller package. At that time in history, it really was a trend setter with no real competition.
I feel the 9mm Shield is the modern equivalent.
The only downside to the Shield in my opinion is the lack of supply. I really hope S&W will work on that.