To be fair, Bungie skimped on the single player to focus on the multiplayer and community aspects. You can't say Halo 3 is rubbish because the campaign has no longevity when theres that level of online features - the normal online, the video maker, the map editor, the co-op.
Maybe if it was something like Bioshock or Metroid that had a 6 hour campaign and nothing else to offer. But Halo 3 was not developed to be a primarily single player game. I think alot of the time single player is the focus and multi is tacked on - it felt the other way around to me with Halo.