There seem to be two forms of DLC. The first is exemplified on the page opposite: a new
landmass and storyline that feels like they've at least followed the spirit of the original game. This follows the more disappointing trend of sticking you in enemy-filled arenas and telling you to fight for your life.
Key to that disappointment is the shooting: it was only ever fun in Borderlands, never exceptional. The game needed its RPG side to shine, and that's exactly what's dropped for gunplay here. You teleport into arenas and fight ever-increasing waves of enemies. In order to progress to harder, multi-level challenges, you need to prove yourself in battle first.
Early battles pit you against the wretches you typically fight in the opening levels of the full game: simple, boring tussles that show up how much Borderlands needs its compulsive weapon collecting to work. Shooting people and not having loot drop is very hollow. You may later fight a band of Badasses (elites), but they'll end up battling each other and nullify the threat. Then the bosses you've vanquished return to give you more trouble.
Moxxi pushes Borderlands just a little too far from its roots. It was a great game, but I don't know anyone who raved about the boss battles or arena fights. Having all the risk without the reward (until the end) really takes away what was special about the game.
Moxxi is pretty, polished, and as characterful as Borderlands ever was, and it adds a bank to store loot you might want to keep, but it's really not something you should spend your pocket-money on.