I feel like I am a basically saying the same things over and over but Thorn is fantastic for both PVP AND PVE. It handles extremely well for a hand cannon, has 9 shots in the magazine (the last of which does bonus damage), penetrates targets (so you can often kill more than 1 mob per bullet) and does Damage over Time to anything that you didn't kill with one shot.
Versus a level 30 enemy the last bullet does 2250 on a crit and then the damage over time portion is 80-90 a tick after that.
Datto did a video recently where he compared Thorn, Hawkmoon and TLW and gives lots of information about all three guns. Bottom line, they are all great and Thorn could easily be your primary gun in both PVE and PVP.
https://www.youtube.com/watch?v=6PEI9ZHRZBU&feature=youtu.be&a