I beat it at like 1:30 this morning.
Right now, I'm fairly disappointed with it but I think that will fade once I start playing it again on harder difficulties and start searching for easter eggs and crap.
HOWEVER, I am thoroughly upset with the complete lack of level exploration. In Halo 2, every map was a playground for me, especially New Mombasa. I could scale that map better than SpiderMan himself could.
But in Halo 3, There is absoolutely NO exploration. You go from Point A to Point B with little to no room for exploration.
It's just so linear.