As much as Borderlands 2 is improved over Borderlands, I found it was less repayable, mainly because the focus of the game is loot, even though the plot is a huge deal. You play the plot, but you play to get more powerful, and after you reach the highest level, even with only one character, it seems to lose all replay value, at least for me. :(
The loot generation system is more strict, so most powerful stuff at the end of the road isn't very diverse or fun, at least not compared to the first one.