Variety is the spice of life, too bad Gearbox didnt keep this in mind when making Mad Moxxis monotonous arena combat.
The dlc, which you can fast travel to, takes you to a circus-like tent that acts as a hub area, which contains a welcome addition of a bank vault, where you may store excess gear, which is a useful thing to have, but you may not need it if you have saved all the claptraps on your first/second playthroughs of Borderlands story mode.
But the main reason to travel to the underdome, is of course for the combat arenas. there are 3 in total (unless you count the bigger challenge arenas), the first is Hell-Burbia, which looks like a mix of Old Haven from the main campaign, and Jakobs Cove from the zombie island dlc, the second, Angelic Ruins is remeniscent of the site where doctor Tannis's research lab is, and the third, The Gully is mostly like a less sunny Kroms Kanyon.
Whilst in these arenas, your play time is solely devoted to fighting off wave after wave of attackers, which may include Bandits, Skags, the Crimson Lance, and any number of Psychos baying for your blood, unfortunately there are no new enemy types to shoot at, and the environments all hark back to the main game, making this dlc feel rather pointless, seeing that there was also a couple of combat arenas in there too.
But what makes the combat in Mad Moxxi interesting is the conditions that are imposed upon the waves, for instance the damage done with one weapon may increase, while others decrease, you may also be required to kill an enemy to refill your health, as it drains between kills, additionally in later rounds these conditions can stack up, with 2, 3, or even 4 used to help/hamstring you.
The waves themselves come in batches of five, which make up a single round, the first wave is the starter wave, the second is a gun wave (all enemies have guns), the third is the horde wave, which consists entirely of Psychos and Midgets (no guns), the fourth is called the badass wave, because there are badass baddies in it, and the boss wave contains a boss/lesser boss from the main game, for instance sledge, Baron Flynt, and King Wee Wee feature, with a few of their goons thrown in for good measure by Mad Moxxi.
Mad Moxxi is the titular female owner of the underdome, who sits above the penalty area, which players go to when they die during a wave, and commentates on the action unfolding below her, in an occassionaly funny/kooky way, unfortunately after 2-3 rounds you will have heard most, if not all of Moxxis commentary, making hearing it more seem so annoying you may consider Moxxis cleavage her only redeeming feature.
Another relatively redeeming feature of the dlc is the reward of two additional skill points, this seems small but to those on their second playthrough this may come in handy, especially if you have hit the level cap of 50.
However the difficulty of the underdome is not determined by a players level, but by their story progress, making it equally difficult for newbies and fifties alike, unless you team up with a buddy you may well struggle through the waves, which finish with ammo drops (which bounce wildly away from your reach at times), and the rounds, which end with around 4-5 weapons spawned, which may or may not be useful.
Apart from this there are no other rewards such as money, and ammo dropped by your assailants, you also cannot increase weapon proiciencies, or gain experience from killing enimies, though you can progress towards the in game challenges, this is hardly consilation given the sheer amount of relentless, tiring combat you have to go through to finish the dlc.
In my opinion the larger challenges are a step too far (around 10 rounds too far), they are 20 rounds long, which totals 100 waves, and over 1000 enemies you will need to kill, and there are three of these, this length of combat seems to serve the purpose of extending the play time of the dlc, but unfortunately will bore even the most bloodthirsty of adventurers after a couple of hours, and long before the 20 rounds are up.
If Gearbox had included more arenas, and not just three, as well as new enemy types not seen in the main campaign, with no challenge lasting more than ten rounds, and more experience/loot gains this dlc may be allot easier to reccomend, but as it is, at the price it is being sold at, Mad Moxxi is a waste of time and money.
Given that The secret armory of General Knox is on offer for less cash, while offering so much more, content wise, this is a bad deal, so unless you really want the trophys/achievements, or are a diehard Borderlands fan, stay well away from Moxxi and her underdome rip-off.