I definitely agree that it's underrated. It's actually my 5th highest kill gun. That doesn't necessarily make it the better all around gun though.
The saiga often takes 2-4 shots to kill with only a 10 round clip. It has pretty average range at best, and an absolutely horrendous reload time. But it is very good at clearing buildings of groups of enemies. It's also far superior to the KSG if you plan on hipfiring often.
The KSG has somewhere around double the range of the Saiga, more ammo, can one shot killstreaks like the guardian and sentry gun due to its insane damage which also makes it extremely deadly with an MMS because it maintains enough damage through cover to still 1 shot people regularly.
There are only a few situations where the S12 is the better option. I love the gun but i can admit it's not quite as good. The S12 is definitely more of a "noob friendly" weapon due to its ability to spam bullets but in the hands of a skilled player the KSG is usually the better option.
"Persistence until excellence." http://www.youtube.com/user/supercoolisaac