As bad as the library was in Halo: CE; Halo 3's Cortana level is even worse.
As for Halo's campaign...it has incredible levels...and then has terrible levels IMO. I loved playing through Sierra 117, and the Covenant/Ark levels were incredible halo fun...but then Cortana exists to drop down the excitement...though I must say there is more good than bad to the campaign IMO.
Having decided giving Halo 3 a shot I started looking for a summary of Halo and Halo 2, just to get the story straight, and I found this: http://www.breakitdownblog.com/halo-story-so-far/
Thought maybe it could be of interest to someone else too.
Why does everyone think that Cortana is such a terrible level? I don't think its the best one in the game, mainly because I'd rather have bigger more open feeling levels then going through cave like areas but its not THAT bad. Comparing it to the library or guilty spark in the original Halo is a joke... it beats them by a mile