I’ve been running a Magpul Backpacker stock on my PC9 since last year.
I love the compact form of the rifle taken down into a little cube.
The one drawback for me is the short forend. I really prefer to run my hand a little farther out and mounting a light to the Mlok has the light too far back and you get a lot of barrel shadow in the light beam.
I’m debating trying a Midwest industries chassis system to get a more AR like form factor for better ergos and control of the weapon.