The only playable level is Delta Halo but even that one sucks compared to Halo: CE and Halo 3's levels.
Halo 2's levels are mostly dark and full of flood. And you have to play as the enemy 50% of the time which makes it even worse.
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Halo 2 and 3 aren't really known for their campaigns. I enjoyed 3 more than 2 on a first playthrough, but don't really enjoy going back to either to be honest. I prefer Halo 3 just because of the slow motion cinematics of Cortana and gravemind and the soundtrack. edit: didn't realize I already made posts here. [Edited on 05.01.2012 7:14 PM PDT]