"Shawty got legs for miles!"
When compared to Pirate's Booty, Campaign of Carnage, and Big Game Hunt, Dragon Keep has both more content and more general polish. There are a bunch of of substantial zones to explore, and each is filled with plenty of new and interesting enemy types. And of course, the writing and voice acting is as excellent as ever.
There are also a ton of sidequests -- and you should absolutely do them. Here's the thing: I didn't. I wanted to run through the story quickly, so I didn't attempt any of the sidequests, and ultimately I realized that it would have taken about the same amount of time; because the main quests take you through almost the whole map anyway. Just, without sidequests punctuating the journey, there are long stretches of very little happening. So yeah. Do the sidequests.
Even though I stupidly missed out on much of what this DLC has to offer, what I have seen gives me the confidence to assert that this is Borderlands 2's best DLC yet. Other than a moment toward the end where the plot gets a little too ... heavy, the outright comedy on display here is as good as you could possibly expect. And the combat is legitimately some of the best since the original game, with an incredible amount of enemy variety.
You should probably bring some friends, too, because many of the encounters are super-difficult. I had some real trouble near the end, despite being substantially above the enemy level range.
Better than: Borderlands 2: Captain Scarlett and Her Pirate's Booty
Not as good as: Borderlands 2 on its own
Seriously: Do the sidequests. I cannot stress this enough.
Progress: Finished the main quest.