Gamespot has given Knoxx a 9! You can read their review here – in case you were on the fence, it looks like the reviews agree: Knoxx is our best DLC yet!
"Rekindling is what The Secret Armory of General Knoxx does best as it embraces all the elements that make Borderlands so fun and addictive. And, it will likely keep you going for at least 10 hours. Whether you've just unlocked The Vault or maxed out twice over, The Secret Armory is an excellent reason to journey back to Pandora."