Halo 2 took everything that CE did well, and made it even better. It's an improvement in basically every way; better gameplay, better story, better environments, better ambiance, better weapons, more and better characters, and they're more fleshed out (even cannon fodder like enemies and marines have more character to them), etc. The difficulty is amped a bit, it's just hard enough at times that reaching the end of a mission that killed you a dozen times over is more gratifying than anything. Overall, the Halo 2 campaign (also) holds up very well, and it's a fun ride the whole way through. I'm glad I managed to stay spoiler free of it for so long.