My purchase of Halo 3 yesterday was the first time that I've ever bought a new release console game on its release day. I picked up the Limited Edition and here are the specs of the extras that are included with the game disk:

Bestiarum - An illustrated hardcover guide to the species and cultures of the Halo Universe

"Essentials" disk which includes:

Featurettes - a series of short films with subjects ranging from making your network play nice with multiplayer, to the challenges of building the biggest Halo ever.

Art Attack - exclusive Halo 3 Gamer Pics and Dashboard theme; gallery of concept and production art; a fully playable version of a classic Halo-themed mini game.

Hi-Def Halo - A Halo-themed calibration tool helps you make the most of your home theater experience. This bonus disk as an Xbox 360 software title, displaying 720p HD Content (on HD sets) for every segment.

So far the story is very engaging and I've also been able to try the multiplayer out a little and I like what I see so far. I try to get my gaming time in before my kids get home from school and after they go to bed, because they take it over in between.