"Rewinding several years many people were excited by the prospect of Halo: Chronicles, the ill-fated Peter Jackson project to be developed in conjunction with the Halo film.
After funding fell through for the movie, it's widely known the production team weren't prepared to just part ways and pretend it never happened; instead the team moved onto making District 9 which was released recently to critical acclaim.
What isn't commonly known is that Halo: Chronicles was put in an identical situation. With Halo: Reach under development since Halo 3, when Chronicles was dropped alongside the film what was there to do for a full team in Bungie that wasn't needed yet in Reach's development but had about a year and a half free time? Why, make a spin-off of course!
To make this review more useful, I'll be splitting it into reasons to buy Halo 3: ODST and reasons not to, so you can make up your own mind as to what's important for your tastes."