It's tired cliché that you never get a second chance to make a first impression.
Despite rampant accusations of having a story that's derivative pulp, the first half of Halo 3 is not a tired cliché, nor does it need a second chance to make a first impression.
Hearing from a number of reviewers and community members who played through the game in a single sitting under Bungie's purview, I've decided to play no more than two campaign levels a day in order to savor the experience. To get my Halo 3 fix the rest of the day, I watch films of those levels, hunt for skulls, or play around in Forge. I understand why those people wanted to get through the game right away, and why Bungie wanted reviewers to play the whole game. Community members knew they wouldn't be able to go on much longer without someone spoiling the ending for them. Bungie knew it would be best for reviewers to have a complete experience-- at least, the most complete experience you can get from 8-15 hours of a game that probably has as much if not more replayability value as Halo 1 and 2 combined. However, I refuse to be rushed.