I'm not a fan of Millar, but i picked up DK returns TPB and DK2 just to see hype and see why people say DK is their top books.
I don't think DK returns is that special, the only really good bit was towards the ending. What Millar have here is a good idea having an aged hero return, and Millar could use this story on any superhero character. But storywise is nothing special. Maybe people saying it's their all time fav book, it's because it's batman, or because Millar wrote it.
Now DK2, i don't know why people are complaining about Millar's work here. I never read Sin City, so don't really know Millar's writing style. But this first issue seems quite interesting and going at a right pace. Also i like how the characters are much the same from the first DK book. What i don't like from this book is maybe Miller's change of artwork. In the first DK the drawings were more detailed, whilst DK2 seems a bit rushed, but i guess Millar wanted to change his style. I still think this a good book and would get the following issues.