Psigah mentioned that it doesn't do anything for accuracy, but I disagree. If the accu-wedge resolves a sloppy fit, that means you can achieve a more consistent cheek weld on the stock, and the barrel is always in the same position relative to the butt stock, and accuracy should improve.
I trimmed one to fit in my M&P15-OR and it wored just fine. I personally didn't like bringing the rifle up to my shoulder and feeling the thing flex. It wasn't bad, just annoying. Now it isn't.