I like (double)shotguns very much. The point in shotguns is, as some have already pointed out, the area of effect. Every time you have multiple enemies in sight you should aim to hurt the most of them with a single shot, and if possible, taking cover to reload, at least before shottyman. They also autohit everything, which is a nice thing.
Once you have killed 10-20 lost souls with a single shot you just can't resist shotties anymore (at least I couldn't after my all-you-can-eat exp fun at the City of Skulls with a Shottyman)
Shotguns are also much less vulnerable to armoured enemies. As many have also pointed out, shotties aren't that effective at longer ranges. For those I usually have my advanced RL, which gets a nice benefit from Reloader, making it a much more effective weapon. Also, if there's a group of enemies out of sight, doubleshotty can be useful, as you will be hurting all of 'em at the same time. I've killed lots of arachs at those nasty caves like this (they seem to hang around in gangs every now and then).
Are shotties actually more useful than CGs?
I don't know, I just use 'em 'cause I like 'em.