The last game I purchased was Halo 3:ODST. Even though it is just an expansion, I believe it is a good value because you get every map released for Halo 3, a campaign and a completely separate multiplayer mode. While the campaign was short, it was a good length for an expansion, and I thoroughly enjoyed the story and presentation.
Graphics are the best they have been in Halo, although they never have been able to do human faces too well...I really enjoyed the Gameplay enhancements, including the revamped HUD, and I'd say the mechanics are much better than in Halo 3. I wish the Firefight mode (Horde equivalent) had more customization to it. I would love to have enough control that I could fight one hundred grunts unarmed, and then a bunch of Hunters.
But sadly it's contained to predefined waves of enemies, and an ordered skull schedule. the waves will also continue until you die, so the best you can come to "beating it" is to unlock the achievement for gaining 200,000 pts on each map. Overall I'd give it a 9/10, and I am very much looking forward to Halo Reach (prequel to Halo 1, with many Spartans) If anyone plays on xbox live feel free to add me Mr Panucci