The DMG2 features advice and rules for Dungeon Masters of all levels of experience, with a particular focus on running adventures and campaigns in the paragon tier (levels 11–20). It includes advanced encounter-building tools (including traps and skill challenges), storytelling tips to bring your game to life, new monster frameworks to help you craft the perfect villain, example campaign arcs, a comprehensive look at skill challenges, and a detailed "home base" for paragon-tier adventurers—the interplanar city of Sigil.

Here are some other reviews from around the Internet.

The Newbie DM

"The Dungeon Master’s Guide 2 is a great book.  Go get it.  Go buy it today"

Stupid Ranger

"This isn’t about who wins; it’s about having fun.  So if something is impeding the fun, discuss it and see what compromise is available so that everyone is happy."

You can find more but I thought these were good representatives. It comes down to this. The DM can make or break a game more than any other player. They are like the QB or the Pitcher. They handle the ball first on every single play. The DMG2 spend some quality time with you on being a better DM. It focuses on some great gaming examples to help drive the ideas through your head in practice. I've seen no bad reviews yet 

I've got mine ordered and it will be here next week!