It started out alright, and I enjoyed the Arbiter levels. Then it started jumping around and introducing new characters, and the Gravemind was just a disaster. Then it ended alright.
I personally feel that Halo 2 and 3 were largely over rated. They just rode the hype that was combat evolved. Now I believe Halo 4 will be a fresh restart to the series, but I also have a fear that 5 and 6 will be largely over rated.