I hate to be negative about a game I love, but here's my rant on the Legenary AI:
One of the things that made Halo great, IMHO, was the fact that the AI didn't cheat. Enemies could be outsmarted, they made mistakes. On Legendary, they were tougher, but not impossible. I never felt that they cheated.
In Halo 2, they cheat, plain and simple.
What's cheating? Playing in ways that violate common sense. When I approach a corner and see Marines dodging sniper fire, with shots landing on either side of them and occasionally missing I know that if I poke my head around the same corner I will die, plain and simple, in half a second. Snipers never miss when firing at the MC, even when driving in a warthog 100 yards away while doing massive jumps and slides. One shot, dead. The Marines not only dodge fire, but take multiple shots to kill. It's not fun, not fun at all.
I've seen similar behavior in vehicles, Elites and bosses. It's the equivilant of the 'rubber band' AI in racing games: You get so far ahead and the guys behind you suddenly catch up. It's lazy programming - and it takes you right out of the game.
I love Halo dearly, more than any other game I've ever played, but Halo 2 is making it harder and harder to love. With nearly every other aspect of the game superior to Halo, it just plain hurts to have this impact the fun so much - but it does.