I've never seen anybody use it in PvP. Honesty, I'm a little afraid to even try it. I've been building mine up lately and j think it's a lot of fun. It even kills more quickly than my Red Death. With that crazy high rate of fire, it seems like it'd be sort of doable in PvP.
I've heard some people say they like using Dreg's Promise, but I haven't received that bounty yet.
Edit: I tried it out a couple times in PvP. Coincidentally, those matches were almost 100% shotguns, so it was interesting. It makes quick work of shotgun runners, but it's going to take practice to get the aiming down. The times I was able to aim it well I managed to take them out almost every time. The problem is they only need to get you in the crosshairs for a split second, while you need to keep on them for multiple shots. It's easier said than done, since it all happens so quickly at close range.
Edit 2: I'm sold. This thing kicks ass once you get used to it.
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Trying to complete sidearms grimoire 5000 kills using Vestian. Dregs Promise is such a frustrating gun to use so would leave it till last of the three ciphers.