It's useable because of the PvE slug buff, but it eats your exotic slot, so it's not something you'd want to use unless you're desperate to use a kinetic slug and have no other options. Ideally, you'd want to grab a Heritage from the Deep Stone Crypt raid with Reconstruction + Recombination + Assault Mag for maximum DPS. Auto-Loading Holster also works. Chaperone is more tuned for PvP. You can OHK people at such ridiculous ranges that it feels like cheating, but only if you're dead-on accurate with slugs.

All slug shotguns are great damage wise, though I personally wouldn't trade out any of my legendary slugs for it in PvE. I would rather use my exotic weapon slot on something a little more useful in my opinion. A great rolled Blasphemer, Heritage, or FILO would be more viable in my mind than Chappy.