20 bucks is pretty cheap for the sheer quality and quantity of the content. I even consider the length of the content to be much longer than shivering isles, because it adds several elements that drastically alter gameplay, making the old content feel fresh again.
I agree, althought the quality is up for debate. A lot of bugs are coming to light, some of them that are game wrecking. The DLC is good though, I would honestly say it is my favorite DLC that Bethesda has ever made (including major ones like Shivering Isles). The DLC had a great story and that is what sold it for me. If they want to continue to make good dlc, they need to sell us on the story first, because the gameplay doesn't really have a lot of ways to expand. I also agree with those saying that the DLC probably should have had some more extra stuff, like weapons, armors, etc. For how little real estate they added in this expansion, it wouldn't have hurt to add more unique items, but that is a small complaint, as the stuff put in is pretty decent.