For me, the most glaring issue with Madd Moxxi's Underdome Riot is the fact that you do not gain experience from combat. You know those sweet sweet numbers that pop up when you defeat an enemy? They're gone. Sure, you still get experience points for turning in quests, but part of the stick on a carrot that drives me in this game involves constant progression, and that's missing here. Call me needy, but this omission is really enough to make me weary of investing a lot of time into this content. It wouldn't be quite as painful if there were interesting quests to complete, but as it stands, the quests are as vanilla as they get. They're as simple as completing each tournament, although that's a feat easier said than done. Speaking of that...
Tournaments within the Underdome Riot don't seem to scale for solo players. I realize most are going to play this with friends, but sometimes I want to play the lone wolf. Sometimes I don't want to be social. Shouldn't we be at a point in gaming where the difficulty dynamically adjusts to the number of players? Sadly, if you want to make a whole lot of progress, you're going to need real-life Internet people. The difficulty, even with four people, is certainly up there with the best (or the worst depending on how you look at it). Compound that into something for one person to deal with, and I wouldn't be surprised if you never saw the other side of the first boss round.
Ok, so it's tough. I'm an experienced gamer, and I can take it. After all, the developers obviously created this DLC with teamplay in mind, so I might as well enjoy the content as intended. If only there was some way to get into a quick game with complete strangers. Sorry. You're out of luck. No separate lobby for Madd Moxxi. You're definitely welcome to use the matchmaking already built into the game, hope you wind up with three other players that have also purchased this content (likely a mistake), then fast travel to the Underdome, but that really seems convoluted. Why wouldn't there be a way to facilitate group play a bit better? This should be a pick up and play game mode, and it doesn't seem like a whole lot of attention was given to that idea.
The fact is: I just don't have the time to try to play something I don't like. What should have been a no-brainer on the design end of things was not. I loved Borderlands and its first DLC: The Zombie Island of Dr. Ned, but Madd Moxxi is another monster altogether. It has charm, but not much more. Madd Moxxi could have been an excellent character to add to the Borderlands universe, but she, along with her arenas, fell flat. It feels like a waste of time and money. I'll cut my losses here and now. I'm shelving this game until something better comes along.