So as you've heard everyone else say, HALO 3 starts right where HALO 2 left off. Master Chief is embroiled in the conflict to end all conflicts aiming to be the savior of the universe.
First off, the story is... just a continuation/conclusion of the HALO saga. Don't really want to say anything here because this is a review, not a walk through.
When you first start out, you might be a tad bit disappointed with the graphics as I was. After the first 15 to 20 minutes, however, you really begin to appreciate the visuals. There is SO MUCH STUFF that is rendered on the screen at any given time (eons more than GoW), and all of it is really very detailed. The only stuff that disappoints is the character models... very generic and last gen.
The sound is... wow. It's a huge part of the atmosphere of this fantastic finale. While you are playing through for the first time, my bet is that you won't really notice it all that much, but take the time and listen. You have to appreciate the fact that you are not only getting a great game, but an entire orchestral musical score made specifically for it as well.
The game play is classic HALO style run and gun. Nothing really all too new or fancy except the new "deployables." These X button wonders are really a lot of fun to play around with and there are found in a good variety. During my play through on heroic (PLEASE DON'T START THE GAME ANY LOWER THAN THAT!!!), I need to use them quite often. They were pretty much essential for me.
*BUT* Who buys HALO just for the campaign? The proof is in the pudding... oh, excuse me, I mean the proof is in the multi-player! That's where the gold is. It's a (much) bigger, better, and shinier version of HALO 2's amazing frag fest, and it certainly is no slouch. Lotsss of maps, weapons, vehicles, and death! Did I mention the "Forge?" Well, I wouldn't want to write another 5 paragraphs (and you wouldn't want me to either I bet).
So stop reading my (fantastic) review, and go out, or in, really, and PLAY!