Just something that I posted on S&W forum in response to carrying a full size M&P:

I have a 9c and 9fs. I carry them in a Crossbreed Supertuck Deluxe. Obviously the 9c is easier to conceal. However, as of late since summer hasn't been too hot (mid 80's) I have been carrying FS to see how hard it is to hide.

So, my conclusion is that it can be done, however like others have said, it really does depend on your build, clothes (loose shirt is required), and the grip is the main problem to overcome.

For me, it works fine as long as I don't have to bend over significantly or I am not sitting in chair with an open back like in a restaurant (I am worried about printing), otherwise concealment is fine.

The looseness of the shirt is a big change for me. I am used to OWB with a cover garment, but it is a little warm for that. I found that most of my t-shirts, while far from tight, are a little too snug for the full size grip. Now, the 9c? In the same holster it hides easily with everything but tight shirts and I don't have to worry about bending over or sitting.