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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While Halo 2's campaign is my least favorite out of 1,2, and 3's, I wouldn't say it sucked. It just didn't draw me in as much is all.