Perhaps one of the biggest reasons for Halo fans to pick up ODST was the three exclusive multiplayer maps, Citadel, Longshore, and Heretic. Buying Halo 3: ODST is the only way to get your hands on the maps, until February 2nd. That's when the Halo 3 Mythic Map Pack 2 will be available for purchase, packaging all three new battlegrounds together for the standard map pack price of 800 Microsoft points.
As an example of how much of a Halo 3 multiplayer fan I am, I purchased ODST, played through the single player, and then traded it back in without giving the additional maps any thought whatsoever. I suppose I'll be able to catch up come February 2nd.