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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Yep, Halo 2 is pretty fail guy that afraids of good story, good gameplay, and good level design.