I just like that fact that I'm an ODST soldier. But even though both games have Firefight, yes, less people will play it. But how many people do you see playing the game for the good of just playing the game? Almost every single thread in this forum is people looking for players to do the Vidmasters.
I think this game will serve the same purpose it does today. To get Vidmaster achievements. I though, play the game to play the game. When Reach comes out, I'll be playing that a lot. But sometimes I might decide to pop in Halo 3: ODST.