I got the DMG2, and I think it is a way superior book for DM'ing than the first one. While the first book had all the rules necessary to teach you howto craft encounters, add monster templates, and hand out treasure, the DMG2 takes it all in a different direction.
The book is chock full of good, solid advice on crafting stories, using your player's preferences to create adventures tailor made for them, and encouraging roleplaying. It also has a more in depth look at skill challenges and how to run them.
You can find my full review over at my blog.