Having played all but Reach (and not religiously), I've never been particularly impressed by the AI in the Halo games. I didn't really find Halo 4 to be any better or worse overall, except the AI trying to fly banshees was just horrendous. 99% chance they'd get stuck on a wall.
The AI in terms of like running around is not great, but the grunts are absolutely fabulous. "you killed my brother" and he charges u 2 glowing stickies, its great.