How is it in comparison to Booty and Moxxi's campaign? And is there a new Invincible boss?
I like it more than the Pirate DLC and way better than Moxxi. Unlike Moxxi, it's mostly a normal quest-based DLC with a few arena missions thrown in, and those arena missions are about as short as 2-3 rounds in a slaughterhouse. The characters are much better (Torgue has some of my favourite lines in BL2, and Tina has much better ones than in the main game). Like the Pirate DLC, there really isn't any new loot worthwhile besides a seraph item or two.
There is one new invincible boss, he seems like he could be easy because of how slow he moves but I never beat him yet because I never noticed the water valves you use against him and ran out of ammo trying to get his shields down.