Halos AI isn't overly brilliant, sure, the enemies hide when you shoot at them, but that's pretty standard nowadays, there's no sense of the enemies working tactically against you.
For some excellent AI check out Brothers in arms and F.E.A.R. In both games the enemies will actively attempt to flank you, and recognise when you are doing it to them, and react accordingly. There's been so many times in those games that I've moved to attack an enemy on his flank only to get filled with lead from behind. In F.E.A.R moreso, I often find myself having to fall back through the level as the enemies attack you aggresively, but not stupidly, they move in pairs so you can't just pick them off, and always laydown cover fire for each other.
F.E.A.R also has a much better atmosphere than Halo, it is a genuinely scary game!