I honestly can't understand why this game isn't more popular with the Freaks crowd - words simply cannot describe just how fun and addicting this game can be.
Case in point, they've already released the first DLC pack called the Zombie Island of Dr. Ned. I played through it on the weekend and had a ton of fun with it. There are new monsters (including numerous different types of zombies), new bosses, new map locations to explore, and a completely self-contained storyline that can be completed at any point during the main campaign. The whole thing took a good 5-6 hours to complete, which isn't shabby for a $10 download.
To top it all off, Gearbox has already announced the next DLC pack, which is slated to be released on Xbox Live on December 29, 2009. It adds three new Thunderdome-type arenas to the game, as well as storage containers for holding all your rare weapons and the ability to unlock two extra skill points to really overpower your character. I can't wait.