Remember, Halo is ment to be played on a HDTV, 1080p, HDMI, with 5.1 surround sound to fully have the experience.
The retail price is not worth it.
If your a hardcore campaigner who really looks in depth of the story you want to play it. Personally though, the Halo 2 EA demo (whatever it was called)seemed more epic and action packed, when your the rookie it's nice and slow and erie atmosphere, looking to the hellish sky, which was good but nothing like the the demo, when the cannon is firing at the buildings, then air strikes being called in. It gives a different prespective, but not that much broader prespective either, just a ODST team, like Delta in Gears Of War.
Instead of telling you what you already know and what others haven't already told you, just simply wait for a price drop.
If your gonna buy it mainly for the DLC, don't. They do have the new maps which couldn't be played unless you had the multiplayer disc which comes with ODST up on market place.