We're starting to see reviews roll in for The Secret Armory of General Knoxx, and they're looking pretty awesome so far. Here's IGN's take; they've given Knoxx an 8.8!
"If you enjoyed Borderlands and still wanted more, then definitely download The Secret Armory of General Knoxx. The main story line is still a blast even if you go it alone and some of the side quests will challenge the most hardened team of Boarderlands veterans. This is by far the best of the three add-ons for the core game, and the most fun I had since I completed the main game last year."