I don't remember anyone saying the campaign was gonna be as big or bigger than Halo 3, just that the game got bigger than they originally intended. Also haven't had that disk load error.
Anyway, I have it and have enjoyed it. I've only gone through the campaign once but I've played Firefight a lot. If you like Halo, it's definitly worth getting. I'm not so sure it's for those who don't like Halo. I'd say, if you can, try Firefight but otherwise it's pretty much a Halo game.