As a player who LOVES playing a Shotty build on all occasions, I have to answer you this: A Shotty build is not only a fun idea, it's a good idea.
Shottyman makes the Combat Shotgun into not just a transitionary weapon from Pistol to Chaingun, but to one of the finest examples of semi-auto firepower known to the game.
The Double-Barreled Shotty will be your mainstay in almost all combat situations, with MASSIVE power in those two dinky little barrels. Just be sure to have Shottyman before using it exclusively, after all, the reload time is a pain without Shottyman. The main thing that makes the Double-Barreled Shotty so effective is not just the fact that it does so much damage, but the fact that non-human enemies seem to enjoy getting closer to you as you shoot at them, allowing you to dispatch them easier and easier as they get closer and closer.
Your main threats in DoomRL are Captains and Commandos, with the Plasma and the Chaingun. They can often hit you better from far distances better than you can hit them, and so if you have a wide open space between you and them, you're likely to end up toast unless you get a lot of luck. Very few other things offer any real challenge, as Hell Knights usually fall within 2-3 DBS close range shots, and pretty much everything else just doesn't pose too much of a threat.
One of the key things to remember about a shotty build, however, is that you cannot, and I MEAN cannot, rely on just your shotguns to pull you through everything. Sometimes, a chaingun or a rocket launcher is almost needed in order to effectively dispatch your foes, and putting a point into Eagle Eye helps with this. In the game I'm currently playing, I have the Minigun, and it has helped me out so much you would not believe it, despite the fact that people say it cannot hit a single thing.
Uniques to CERTAINLY look out for:
Assault Shotgun: This is the second best unique for a Shotty build to use, the reasons why are simple. Shottyman works with it, and it doesn't load just one shell, it loads EVERY shell, automatically. Along with that, there are times where you should just stand your ground and gun. The Assault Shotgun DEFINITELY helps with that.
Pancor Jackhammer: This is THE best Unique for a Shotty build, due to the fact that it is basically a 3-shot Double Barreled Shotgun, AKA: DBS on crack. I have not had the fortune to actually use one of these, but I have absolutely no doubts in my mind that this thing will tear your enemies to shreds.
Gothic Set: You might ask me, "Why? Why would you want to slow yourself down so much with such bulky armor?" The answer is simple: Superior Firepower leads to Superior Results, this is true, but without a solid shell of armor to keep you using that Superior Firepower, you're toast. With the Gothic Armor, you need not fear even Commandos. Although you may move towards them ploddingly, you can laugh as the plasma does a measly 1 hp damage every single time it hits, before blowing their head off with a DBS. Plus, Knockback is your enemy in this game.