Yeah, it's a lot better than FEAR. FEAR had some nice AI, but it still did a lot of random stuff that didn't make sense (they loved diving under tables whenever possible and it often seemed silly). The way the AI in Halo reacts is just really satisfying overall. It's aggressive, but in a realistic fashion. They move around all over, flank you, work in groups, retreat when necessary, and generally just create an interesting series of encounters. It's very rare in the genre these days, unfortunately.
Most games these days tend to use Call of Duty-like encounters with enemies simply taking cover but Halo still features battles out in the open which results in a lot of unique scenarios. As much as I love the Crysis games, the AI is a huge step below the Halo games (especially Reach).