With buckshot/flechette ammo, not that great against most targets (because armor is so very very common). With slugs, they can be a scary in-close option for someone who's relying on a single firearm skill (instead of having Pistols or Automatics or what-have-you, like a dedicated sniper-type). They're a classic on the streets for intimidation value and relative bang-for-the-buck, availability on many is low, so they're often easy to get ahold of (or to toss after a gig if the heat is on). Their base damage code is pretty solid, and they're good for what they're used for IRL (blasting single targets during close-in firefights, like roomsweeping).
Mechanically, there's all sorts of number-crunching out there about how full-auto bursts are scarier, range brackets can make bigger guns more attractive, there's a lack of accessories for them, and on and on and on. And that's all valid, depending on the game, the power level, and the character.
But I still dig 'em, and for the right character they can just "fit" better than some hardcore mil-spec stuff.