Overall, I enjoyed Halo 3. I'm kinda disappointed that I beat it in seven hours, that last of which was spent on the first half of the last mission, stupid flood... I'm glad it's been left open for a sequel, unless the Marathon Prequel thing is correct... I was pretty annoyed by Cortana and Gravemind bumping in too, slowed the game down way too much. Scarab battles were fun, along with meleeing brutes to death. Overall, my least favorite adversary was the Tank Flood, those guys were frickin' bullet sponges... I've only beat it under Normal though, so I need to go back and try Heroic, then pray to finish Legendary. I hate it that I'm not very good.
On a related note, I second Red's plea for the soundtrack. And is it true that I can't play multiplayer on XBL without a gold account?