The Campaign mode ranges from solid to great. It's easily the best one with the possible exception of Halo 1, though that'd be a choice of opinion :/ It's a very well made campaign, great level design, plenty of polish on the animations and visuals, and only a few odd difficulty spikes.
There's some good levels modelled off of old ones, too. The night time mission when boarding the Truth and Reconciliation is there, as is a mission inside a Covenant Cruiser. There's an urban level, much like Halo 2's New Mombasa, as well. But these are just homages, majority of the levels are unique.
My only gripe is they didn't include a Warthog race at the end :/ That was what I hated most of Halo 2... Halo 3 included it again. It's just so embedded in the Halo games at this point, to have a mad dash through an exploding area torn apart by two or three warring factions. Reach not having it is just disappointing to me.