Finished my first playthrough today with a buddy. Gotta say, this is easily the best game I've played all year. Everything I wanted in a Borderlands sequel, I got. Everything that was fun in the first game, I got, and I got it better. Everything that is new is an improvement. I have no complaints. I can't think of another game that has made me laugh so hard so often.
I haven't played it at all by myself, but if there is an argument to be made that COOP-only games can exist and be awesome then this is as good a candidate as I've played. I picked Zero because I picked Mordecai in the first game, and switching up my previous style of hanging back and sniping with running and going melee in the thick of it took me out of my comfort zone at first, but easily became my preferred playing style. I like how much better the characters play off each other in this one and was happy to respec my Zero depending on who I was playing with.
I can see that I will be playing this one for a while.