I was playing some yesterday and decided to just mix it up a bit and use it. I was surprised on how effective it was. I still don't use it much cause the grenade launcher is much more handy but does anyone else use it?

If it's a map without a lot of destructible elements and I don't need smoke, sure. So... not a lot, but yes, it is very effective.

NO

I think the under-barrel shotgun was the last attachment I used and was only to complete the "get all Bronze stars" badge.  Having said that, I was pleasantly surprised at how effective it turned out to be.  
In theory it should be an excellent "best of both worlds" combination of medium range rifle and closer range shotgun for the Assault Class (so unlimited ammo), but in practice if I find myself wanting to get down-and-dirty and mix it up with a shotgun I tend to go for the full on Spas or a Saiga.

I'm all about the smokes when rolling Assault.

The 40mm shotgun shell  is pretty much a  ticket to a free close range kill, and I've been able to use it to take out multiple enemies quickly as well, since you can swap to the assault rifle to mop up. It's handy if you intend to use a 4x optic as well so you don't have to hipfire your assault rifle up close.

In general, I think the shotgun is more useful than the grenade launcher when you're defending. What you don't usually want to do as a defender is destroy walls, sine that's most of your cover, and may be all that's keeping your objectives alive. What you do usually want to do is quickly kill individual enemies protecting a sneaky MCOM charge, who are likely using shotguns of their own.

Hmm...I haven't really used the shotgun underbarrel quite a lot, I usually just use the 40mm grenades or the smoke grenades.  I should give that a try later :)

Nah, smokes or grenades is where it's at.

it can be very satisfying, if you don't use the other ones too much than its a great replacement. But it's not nearly as useful as the grenade, smoke is only so so in my opinion.

I used it a bit early in the game (before the weapon re-balancing / shotgun fixing), but not since.  It worked really well close range, but always found myself missing smokes/grenade and not coming into many situations where the underbarrel shottie was useful.

Just out of interest  - are people finding the smoke grenades to be useful? and how are using them (I.e. what situations)?

I always either use the shotgun underbarrel or the grenade one,so yeah. I like it.

I use the Shotgun attachment so that I can have the tracer darts in my secondary slot, so I don't feel completely useless against vehicles.