Bought it yesterday and took about 6 hours to finish the campaign on Normal. Mind you I skipped approximately all of the fighting in Mission 10. I didn't stop to fight until the Hunters because the door was closed. FYI, you can get the mongoose past the barricade pretty easily.
It was great fun, but I can't really pick out any particular moments that I'd love to go back and play again, even Halo 2 had that. I'll probably start a Legendary run later today, and in 6 hours likely not be done with the 3rd mission.
I don't have Gold right now and don't plan on getting it particularly soon, but even so I've messed around with Firefight and the custom settings. Amazing what you can change. Legendary+Invulnerability+Trigger Happy covenant really shows what the engine can do in terms of pure insanity going on.