I got so into the game that I didn't notice I finished it in one sitting. I started it at around 5PM and finished it a little past 10. At first I was disappointed that it was so short, but someone told me that this was supposed to be part of Halo 2's story which makes sense.
As a conclusion to the trilogy's story, it's great. Everything's here, and the gameplay is great too, with new vehicles and weapons. I just wish that it got more of Arbiter.