Shotgun works well up close and weak at range (assuming buckshot). I'll use assault rifles on certain maps and shotgun on others. It's a different mindset to use either effectively. I ran up behind three enemies with shotgun only to die after killing two. With assault gun, I would have had all three.

I personally hate the pump action shotguns. I'm used to assault rifles and moving to something that slow makes me have to work. Makes it fun to try something different.