I beat it on heroic in just about 6 hours. It was short but I'd say this was the best out of all the halo games so far in terms of level design. Nothing ever feels repetitive, there's a good variety of environments, and each section is pretty large with a lot of detail.
The cutscenes were awesome, especially the opening, and martin o'donell once again made a great score.
the only thing I didn't like was how they told the story through flashbacks. at first playing as the rookie at night was cool but eventually you just want to get into the action again. those sections had pretty much no purpose other than to look nice.
I didn't get into firefight just yet but so far everything else is great