Of the ones I own: Borderlands' Mad Moxie's Underdome.
It was boring, it took far too long (3hrs each arena), every level was the same waves, in the same order, with the same enemies. What did you win if you completed all the arena's? Nothing but achievements. Beating the Short arenas (5 levels) got you an extra skill point and the weapon bank was open to you right at the beginning.
Another thing is the idea of the 'Weapon Bank'. It's pretty pointless! At what point are you not going to be carrying your best weapons with you? Even if your waiting for those weapons you might need to level up to use, you can't possibly tell me you don't have enough room to keep them with you until then. And it doesn't even transfer between characters! If your using a Hunter and find an amazing SMG, your Siren character can't go to the bank and get it later cause she has her own seperate bank! POINTLESS!