Read on to see if Mad Moxxi's going to be worth your ten bucks when it arrives on December 28 on Xbox 360 and January 7 on PS3.
The structure is simple: A wave of enemies enters, you kill them all, madly try to collect the health and ammo refills that drop from the sky, and then start fresh. Each wave is themed, like the Gun Wave (everyone is armed) and the Horde Wave, which pitches an army of flaming bad guys on a collision course with you.
At first, the only quest available in the hub that connects Moxxi's three tournaments tasks you with completing five rounds of five waves in each arena. You don't need a calculator to tell you that's a whole lot of murdering. Luckily, the constant narration and berating from Moxxi and the roar of the crowd help the fights feel like more of a spectacle than just a connected series of battles.
The weird thing is that battling in the arenas doesn't award you experience -- nor weapons. Though the rewards might be delicious if you make your way to the end of an arena (I'm not sure, my poor Level 11 could never get that far), it robs Underdome fights of the feeling of constant character growth, which is really the driving force behind Borderlands. (Update: Gearbox tells us that new weapons drop underneath Moxxi after each round but "the XP thing is intentional after a lot of focus testing. It breaks the rest of the game if you let people crank up too much XP. This is a separate game mode.")
It seems like, even with Moxxi lording over the proceedings and with occasional variables like reduced gravity, playing through all three arenas would start to feel a little grindy after a while. Of course, I'm sure that the whole thing is turned on its ear with the addition of a co-op buddy, and isn't that the way Borderlands was meant to be played in the first place?