I personally have had a bad experience with this "feature" - not on the M&P though..
I slapped in a magazine in a Ruger P90 that I used to have, but this time it did a little more than release the slide - it FIRED. And no, I did NOT have my finger anywhere NEAR the trigger!
Of course, it sent the round down range and ricocheted off the flood.
Needless to say - that gun was traded in that afternoon.