I never really liked the big-guns skill too much but I ended up raising it when my gauss ammo started to run out.
I liked the Laser Gattling Gun. I liked the animation of the enclave trooper I just shot being cut into half and falling to the ground, torso first then legs after muhuwahahahahah.
The Vindicator was just too hard to use because of the lack of caseless ammunition; the only places I could find it was in San Francisco and sometimes at Buster's outside NCR.
In my opinion, though not a minigun, the bozar is the best big gun in the game. It does incredible damage (I was doing like 400 to Frank Horrigan with each burst with Sniper), and .223 ammunition is incredibly common in the game. The only downside is that it needs to be reloaded after every two bursts.
Of course, not much is alive after two bursts from the Bozar.