I definitely went into this game with low expectations. The consensus seems to be that Halo3 + Reach = good, and ODST was meh. Honestly, I enjoyed the story and characters a lot more in ODST than Halo3. Gameplay is essentially the same. It had its annoyances: Backtracking to get ammo/guns, having to kill tanks without the right kind of weapons.
Loved the fact that I didn't have to fight any flood. I find it much more entertaining fighting the convenant. I kinda have a dislike for the game mechanic of increaseing the difficulty toward the end of the game by replacing all the enemies with 'upgraded' or 'tougher' enemies. Aka, the flood in Halo1, the elite guys in gears of war, etc.
All in all a good game and worth picking up on the cheap just for the single player experience.