I think Knoxx sets the standard for DLC quality and value. It truly is a necessary experience to the Borderlands game.
Secret Armory of General Knoxx - 10
The Zombie Island of Dr. Ned - 8
Claptrap's New Robot Revolution - 6
Mad Moxxi's Underdome Riot - 4
I felt like I could give Knoxx a perfect score. My friends and I have spent almost as much time in the areas around T-Bone Junction as we have the original game areas. Not only did Knoxx allow the player to start progressing in levels again, but it added major challenges and achievements to suck the player in.
1. Crawmerax - almost impossible to beat solo, killing him was more satisfying than killing the original Destroyer.
2. New level of weapons, the ever elusive Pearlescent. The first time Crawmerax dropped one I cheered.
3. Well developed gang of villains in the Crimson Lance, the prison guards, and the midgets that pop out of everything. (One reason I downgraded Claptrap was the villains just didn't seem developed enough, it was like they only put a fresh coat of paint on the original ones we were tired of).
4. New vehicles and fun areas to drive around in.
5. The humor is alive and well in the DLC, I think I laughed out loud quite a few times. Zombie Island and Claptrap were a tad on the cheesy side.
I could consider the mission glitches a negative but decided not to. The player doesn't have to exploit them if they don't want to. It's safe to say that all expansions from here on out (at least the ones that cost me 800 points) will be compared to Knoxx as a baseline.