I bought a 8 year old police trade in M&P full size duty pistol a few months ago. It came with only the small backstrap insert and that's too small for me. I asked around here and got some great advice and picked up a Hogue Handall Grip Sleeve Hybrid for it. Wow! It's hard to believe that this really poorly maintained (dried coffee inside the magazine well, barrel handn't been cleaned in forever, etc.) is my current best shooter of my non-target pistols. The Hogue grip with its soft rubbery plastic and the finger grooves fits my hand like its a custom part and has really upped the accuracy.
Just wanted to a) thank folks for the advice a few months back and b) let others know that a $9 part can make a huge difference.