Mad Moxxi’s Underdome Riot is yet another piece of DLC for borderlands. It's very similar to the horde modes in Gears of War games, and the Firefight modes in later installments of the Halo series. But now, it's Borderlands style!
Borderlands horde is a good idea, you can face off against waves of the games crazy enemies, and fight them with the games even crazier guns. And it works well to be honest. The enemies are roughly your level, so they give constant challenge, there's waves like "Horde Wave!" which send nothing but psychos at you. There's random attributes such as low gravity, enemies get increased health, certain guns get more damage, and head shots do extra damage. That adds some variety, but when pistols have increased damage, every other type of weapon has decreased damage, so if you don't have said gun with you, you might as well throw rocks at your enemies.
The Underdome will allow co-op and the enemies will be at whatever level the host is, so make your low level friends host if you want the horde to go easier on you.
In short Mad Moxxi's Underdome Riot is a small blood bath of fun, but if playing alone it can get rather boring, so don't forget to bring you friends along... Or just go outside.