Shotguns are my favorite weapon for snipers to take as a close-quarters weapon, since it works on the same skill. I typically always get a shotgun with the clip or drum loading mechanism, for reload speed, if nothing else. A shotgun provides relatively high damage at a low cost, and the legalities are often lower than assault rifles. Using flechettes is a good way for a low-initiative or low-skill character to engage multiple opponents at once, and for high skill, high initiative users, it has some advantage over SnS or gel rounds if you need to clear a room, but not necessarily kill everyone, since most people with any armor (even a lined coat) stand a good chance of only taking Stun damage. I mention low-initiative, low-skill characters, because I often use a shotgun for my face or hacker/TM characters, since it has clear benefits over a pistol, and it is hard to justify anything bigger with just a 2 skill, for instance.
So, pros and cons. For a sammy? Go with an assault rifle, definitely, and use the shotgun only if you need a Masterkey. For someone who does most of their work in the social or matrix arenas? You could do FAR worse than using a shotgun. For mages? A shotgun or other weapon goes a long way towards convincing people you aren't a mage, especially if you don't go around with arcane symbols on your lined coat or wearing traditional shaman gear, etc. Convincing people you aren't a mage (until you start slinging spells) means you go further down the 'to geek' list than the troll with the assault cannon. It'll only last a couple passes, but that's still worlds better than getting geeked first, right?