Completed Playthrus of Maya 50 and Gaige 50, and slowly getting bored and currently working on Zero (25). As awesome as this game is unless they're playing with friends I can't see why or how people would chose to replay the game upwards of four-five-six-etc. times.
I play mostly with my brother, but I also play a lot solo. My motivation for starting a new game is to challenge myself to see how far I can go without dying. Using the weapons I collect w/ each playthru, it's progressively more exciting with each new game.