Just finished the campaign. I was entertained the whole time, and I am a fanboy of the series in general (I have played since the first one came out), but I am not sure how much re-playability it has. The game jumps around a lot too. In the first two, you will have specific objectives, and have one large map, but here, you finish one objective, cue cutscene, and then you are thrown in to another area.
Also, some of the missions are too much like arcade games. For example, the level where you pilot the broadsword. Halo 1 and 2 were epic, Halo 3, almost as good, and it went a bit downhill from there. Like I said, I still enjoyed it, but the story got a bit more ridiculous as well. Alsmost as if they were running out of ideas.
I feel that the online will be amazing though. I don't know if my sentences are coherent here.