Besides the story that saw the two teams (and worlds) collide, you also get some previously unpublished sketch material by Frank Quitely. Several pages worth. Portions of Morrison's script are also included. Seeing the notes and additional sketches in the margins is a fascinating thing.
The hardcover release of the Deluxe Edition is in stores this week. DC has given us an exclusive first look at those sketches and bonus pages. Check them out.
It's definitely worth picking up, especially if you haven't read it already. And here's another spoiler, I received an early copy a couple weeks ago so I can for sure say it's pretty cool.